{
  "index": "1939-A-4",
  "type": "GEO",
  "tag": [
    "GEO",
    "ALG"
  ],
  "difficulty": "",
  "question": "4. Find the equations of the two straight lines each of which cuts all the four straight lines\n\\[\nx=1, y=0 ; \\quad y=1, z=0 ; \\quad z=1, x=0 ; \\quad x=y=-6 z\n\\]",
  "solution": "First Solution. Suppose the required line \\( L \\) meets the given lines in the points \\( A, B, C \\), and \\( D \\) respectively. Then\n\\[\nA=(1,0, a), B=(b, 1,0), C=(0, c, 1), \\text { and } D=(6 d, 6 d,-d)\n\\]\nfor some numbers \\( a, b, c \\), and \\( d \\). Treat \\( A, B, C \\), and \\( D \\) as vectors. The condition that they be collinear is that the vectors\n\\[\n\\begin{array}{l}\nB-A=(b-1,1,-a) \\\\\nC-A=(-1, c, 1-a) \\\\\nD-A=(6 d-1,6 d,-d-a)\n\\end{array}\n\\]\nbe proportional.\nThe proportionality of the first two tells us that\n\\[\nc=\\frac{1}{1-b}=\\frac{a-1}{a}\n\\]\nwhile the first and third give\n\\[\n6 d=\\frac{1-6 d}{1-b}=\\frac{a+d}{a}\n\\]\n\nRewrite the middle member here using (1)\n\\[\n6 d=(1-6 d) \\frac{a-1}{a}=\\frac{a+d}{a} .\n\\]\n\nClearing fractions\n\\[\n\\begin{aligned}\n6 a d & =a+d \\\\\na+6 d-1-6 a d & =a+d\n\\end{aligned}\n\\]\n\nAdding these equations, we find \\( 4 d=a+1 \\), so\n\\[\n6 a(a+1)=24 a d=4(a+d)=5 a+1\n\\]\n\nThe quadratic equation \\( 6 a(a+1)=5 a+1 \\) has roots\n\\[\na=\\frac{1}{3},-\\frac{1}{2}\n\\]\nand the corresponding values of the other unknowns are\n\\[\n\\begin{aligned}\nb & =\\frac{3}{2}, \\frac{2}{3} \\\\\nc & =-2,3 \\\\\nd & =\\frac{1}{3}, \\frac{1}{8}\n\\end{aligned}\n\\]\n\nThe direction vectors of the lines (proportional to \\( B-A, C-A \\), and \\( D-A) \\) in the two cases are \\( (3,6,-2) \\) and \\( (-2,6,3) \\), respectively. The two lines are given parametrically by\n\\[\nL_{1}: s \\mapsto\\left(1,0, \\frac{1}{3}\\right)+s(3,6,-2)\n\\]\nand\n\\[\nL_{2}: t \\mapsto\\left(1,0,-\\frac{1}{2}\\right)+t(-2,6,3) .\n\\]\n\nThese lines cross the given lines (in order) for\n\\[\ns=0,+\\frac{1}{6},-\\frac{1}{3}, \\frac{1}{3} \\text { and } t=0, \\frac{1}{6}, \\frac{1}{2}, \\frac{1}{8} .\n\\]\n\nIn non-parametric form \\( L_{1} \\) is given by\n\\[\ny=2(x-1)=1-3 z\n\\]\nand \\( L_{2} \\) is given by\n\\[\ny=3(1-x)=2 z+1\n\\]\n\nSecond Solution. Denote the given lines in order by \\( M_{1}, M_{2}, M_{3} \\), and \\( M_{4} \\). Then the equation of the plane of the required line \\( L \\) and \\( M_{1} \\) has the form\n\\[\ny=\\lambda(x-1)\n\\]\n\nThe equation of the plane of \\( L \\) and \\( M_{2} \\) has the form\n\\[\nz=\\mu(y-1)\n\\]\nand the plane of \\( L \\) and \\( M_{3} \\) is given by\n\\[\nx=\\nu(z-1) .\n\\]\n\nAny two of these equations determine the line \\( L \\), so, if we eliminate \\( y \\) from the first two of these equations, we must obtain an equation equivalent to the third. Therefore \\( \\lambda \\mu \\nu=1 \\).\n\nLet the point of intersection of \\( L \\) and \\( M_{4} \\) be \\( (6 d, 6 d,-d) \\). It lies on all of the planes considered above, so\n\\[\n\\begin{aligned}\n6 d & =\\lambda(6 d-1) \\\\\n-d & =\\mu(6 d-1) \\\\\n6 d & =\\nu(-d-1) .\n\\end{aligned}\n\\]\n\nMultiply these equations and use \\( \\lambda \\mu \\nu=1 \\) to get\n\\[\n-36 d^{3}=-(6 d-1)^{2}(d+1) .\n\\]\n\nThis simplifies to \\( 24 d^{2}-11 d+1=0 \\), so \\( d=\\frac{1}{3} \\) or \\( \\frac{1}{8} \\). The corresponding values of \\( \\lambda, \\mu, \\nu \\) are \\( 2,-\\frac{1}{3},-\\frac{3}{2} \\) or \\( -3,+\\frac{1}{2},-\\frac{2}{3} \\), and we obtain nonparametric equations for \\( L_{1} \\) and \\( L_{2} \\) as before.\n\nFor a general treatment of this problem, see D. M. Y. Sommerville, Analytic Geometry of Three Dimensions, Cambridge, 1934, page 184.",
  "vars": [
    "x",
    "y",
    "z",
    "L",
    "L_1",
    "L_2",
    "M_1",
    "M_2",
    "M_3",
    "M_4",
    "A",
    "B",
    "C",
    "D"
  ],
  "params": [
    "a",
    "b",
    "c",
    "d",
    "s",
    "t",
    "\\\\lambda",
    "\\\\mu",
    "\\\\nu"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "xcoord",
        "y": "ycoord",
        "z": "zcoord",
        "L": "linegen",
        "L_1": "lineone",
        "L_2": "linetwo",
        "M_1": "auxlineone",
        "M_2": "auxlinetwo",
        "M_3": "auxlinethree",
        "M_4": "auxlinefour",
        "A": "pointa",
        "B": "pointb",
        "C": "pointc",
        "D": "pointd",
        "a": "parama",
        "b": "paramb",
        "c": "paramc",
        "d": "paramd",
        "s": "params",
        "t": "paramt",
        "\\\\lambda": "coeflambda",
        "\\\\mu": "coefmu",
        "\\\\nu": "coefnu"
      },
      "question": "4. Find the equations of the two straight lines each of which cuts all the four straight lines\n\\[ xcoord=1, ycoord=0 ; \\quad ycoord=1, zcoord=0 ; \\quad zcoord=1, xcoord=0 ; \\quad xcoord=ycoord=-6 zcoord \\]",
      "solution": "First Solution. Suppose the required line linegen meets the given lines in the points pointa, pointb, pointc, and pointd respectively. Then\n\\[ pointa=(1,0, parama), pointb=(paramb, 1,0), pointc=(0, paramc, 1), \\text { and } pointd=(6 paramd, 6 paramd,-paramd) \\]\nfor some numbers parama, paramb, paramc, and paramd. Treat pointa, pointb, pointc, and pointd as vectors. The condition that they be collinear is that the vectors\n\\[\n\\begin{array}{l}\npointb-pointa=(paramb-1,1,-parama) \\\\\npointc-pointa=(-1, paramc, 1-parama) \\\\\npointd-pointa=(6 paramd-1,6 paramd,-paramd-parama)\n\\end{array}\n\\]\nbe proportional.\nThe proportionality of the first two tells us that\n\\[\nparamc=\\frac{1}{1-paramb}=\\frac{parama-1}{parama}\n\\]\nwhile the first and third give\n\\[\n6 paramd=\\frac{1-6 paramd}{1-paramb}=\\frac{parama+paramd}{parama}\n\\]\n\nRewrite the middle member here using (1)\n\\[\n6 paramd=(1-6 paramd) \\frac{parama-1}{parama}=\\frac{parama+paramd}{parama} .\n\\]\n\nClearing fractions\n\\[\n\\begin{aligned}\n6 parama paramd & =parama+paramd \\\\\nparama+6 paramd-1-6 parama paramd & =parama+paramd\n\\end{aligned}\n\\]\n\nAdding these equations, we find \\( 4 paramd=parama+1 \\), so\n\\[\n6 parama(parama+1)=24 parama paramd=4(parama+paramd)=5 parama+1\n\\]\n\nThe quadratic equation \\( 6 parama(parama+1)=5 parama+1 \\) has roots\n\\[\nparama=\\frac{1}{3},-\\frac{1}{2}\n\\]\nand the corresponding values of the other unknowns are\n\\[\n\\begin{aligned}\nparamb & =\\frac{3}{2}, \\frac{2}{3} \\\\\nparamc & =-2,3 \\\\\nparamd & =\\frac{1}{3}, \\frac{1}{8}\n\\end{aligned}\n\\]\n\nThe direction vectors of the lines (proportional to \\( pointb-pointa, pointc-pointa \\), and \\( pointd-pointa) \\) in the two cases are \\( (3,6,-2) \\) and \\( (-2,6,3) \\), respectively. The two lines are given parametrically by\n\\[\nlineone: params \\mapsto\\left(1,0, \\frac{1}{3}\\right)+params(3,6,-2)\n\\]\nand\n\\[\nlinetwo: paramt \\mapsto\\left(1,0,-\\frac{1}{2}\\right)+paramt(-2,6,3) .\n\\]\n\nThese lines cross the given lines (in order) for\n\\[\nparams=0,+\\frac{1}{6},-\\frac{1}{3}, \\frac{1}{3} \\text { and } paramt=0, \\frac{1}{6}, \\frac{1}{2}, \\frac{1}{8} .\n\\]\n\nIn non-parametric form lineone is given by\n\\[\nycoord=2(xcoord-1)=1-3 zcoord\n\\]\nand linetwo is given by\n\\[\nycoord=3(1-xcoord)=2 zcoord+1\n\\]\n\nSecond Solution. Denote the given lines in order by auxlineone, auxlinetwo, auxlinethree, and auxlinefour. Then the equation of the plane of the required line linegen and auxlineone has the form\n\\[\nycoord=coeflambda(xcoord-1)\n\\]\n\nThe equation of the plane of linegen and auxlinetwo has the form\n\\[\nzcoord=coefmu(ycoord-1)\n\\]\nand the plane of linegen and auxlinethree is given by\n\\[\nxcoord=coefnu(zcoord-1) .\n\\]\n\nAny two of these equations determine the line linegen, so, if we eliminate ycoord from the first two of these equations, we must obtain an equation equivalent to the third. Therefore \\( coeflambda\\,coefmu\\,coefnu=1 \\).\n\nLet the point of intersection of linegen and auxlinefour be \\( (6 paramd, 6 paramd,-paramd) \\). It lies on all of the planes considered above, so\n\\[\n\\begin{aligned}\n6 paramd & =coeflambda(6 paramd-1) \\\\\n-paramd & =coefmu(6 paramd-1) \\\\\n6 paramd & =coefnu(-paramd-1) .\n\\end{aligned}\n\\]\n\nMultiply these equations and use \\( coeflambda\\,coefmu\\,coefnu=1 \\) to get\n\\[\n-36 paramd^{3}=-(6 paramd-1)^{2}(paramd+1) .\n\\]\n\nThis simplifies to \\( 24 paramd^{2}-11 paramd+1=0 \\), so \\( paramd=\\frac{1}{3} \\) or \\( \\frac{1}{8} \\). The corresponding values of \\( coeflambda, coefmu, coefnu \\) are \\( 2,-\\frac{1}{3},-\\frac{3}{2} \\) or \\( -3,+\\frac{1}{2},-\\frac{2}{3} \\), and we obtain nonparametric equations for lineone and linetwo as before.\n\nFor a general treatment of this problem, see D. M. Y. Sommerville, Analytic Geometry of Three Dimensions, Cambridge, 1934, page 184."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "peppermint",
        "y": "bluegrass",
        "z": "driftwood",
        "L": "moonstone",
        "L_1": "cornbread",
        "L_2": "marshmallow",
        "M_1": "tortoise",
        "M_2": "sailboat",
        "M_3": "snowflake",
        "M_4": "pinecones",
        "A": "chandelier",
        "B": "lighthouse",
        "C": "buttercup",
        "D": "workbench",
        "a": "paintbrush",
        "b": "scarecrow",
        "c": "watermelon",
        "d": "horseshoe",
        "s": "goldfinch",
        "t": "starfruit",
        "\\lambda": "jackrabbit",
        "\\mu": "campfires",
        "\\nu": "dragonfly"
      },
      "question": "4. Find the equations of the two straight lines each of which cuts all the four straight lines\n\\[\npeppermint=1, bluegrass=0 ; \\quad bluegrass=1, driftwood=0 ; \\quad driftwood=1, peppermint=0 ; \\quad peppermint=bluegrass=-6 driftwood\n\\]\n",
      "solution": "First Solution. Suppose the required line \\( moonstone \\) meets the given lines in the points \\( chandelier, lighthouse, buttercup \\), and \\( workbench \\) respectively. Then\n\\[\nchandelier=(1,0, paintbrush),\\; lighthouse=(scarecrow, 1,0),\\; buttercup=(0, watermelon, 1),\\; \\text { and }\\; workbench=(6 horseshoe, 6 horseshoe,-horseshoe)\n\\]\nfor some numbers \\( paintbrush, scarecrow, watermelon \\), and \\( horseshoe \\). Treat \\( chandelier, lighthouse, buttercup \\), and \\( workbench \\) as vectors. The condition that they be collinear is that the vectors\n\\[\n\\begin{array}{l}\nlighthouse-chandelier=(scarecrow-1,1,-paintbrush) \\\\\nbuttercup-chandelier=(-1, watermelon, 1-paintbrush) \\\\\nworkbench-chandelier=(6 horseshoe-1,6 horseshoe,-horseshoe-paintbrush)\n\\end{array}\n\\]\nbe proportional.\nThe proportionality of the first two tells us that\n\\[\nwatermelon=\\frac{1}{1-scarecrow}=\\frac{paintbrush-1}{paintbrush}\n\\]\nwhile the first and third give\n\\[\n6 horseshoe=\\frac{1-6 horseshoe}{1-scarecrow}=\\frac{paintbrush+horseshoe}{paintbrush}\n\\]\nRewrite the middle member here using (1)\n\\[\n6 horseshoe=(1-6 horseshoe)\\frac{paintbrush-1}{paintbrush}=\\frac{paintbrush+horseshoe}{paintbrush} .\n\\]\nClearing fractions\n\\[\n\\begin{aligned}\n6\\,paintbrush\\,horseshoe &= paintbrush+horseshoe \\\\\npaintbrush+6\\,horseshoe-1-6\\,paintbrush\\,horseshoe &= paintbrush+horseshoe\n\\end{aligned}\n\\]\nAdding these equations, we find \\( 4 horseshoe = paintbrush + 1 \\), so\n\\[\n6\\,paintbrush(paintbrush+1)=24\\,paintbrush\\,horseshoe=4(paintbrush+horseshoe)=5\\,paintbrush+1\n\\]\nThe quadratic equation \\( 6\\,paintbrush(paintbrush+1)=5\\,paintbrush+1 \\) has roots\n\\[\npaintbrush = \\frac{1}{3},\\; -\\frac{1}{2}\n\\]\nand the corresponding values of the other unknowns are\n\\[\n\\begin{aligned}\nscarecrow &= \\frac{3}{2},\\; \\frac{2}{3} \\\\\nwatermelon &= -2,\\; 3 \\\\\nhorseshoe &= \\frac{1}{3},\\; \\frac{1}{8}\n\\end{aligned}\n\\]\nThe direction vectors of the lines (proportional to \\( lighthouse-chandelier, buttercup-chandelier, \\) and \\( workbench-chandelier \\)) in the two cases are \\( (3,6,-2) \\) and \\( (-2,6,3) \\), respectively. The two lines are given parametrically by\n\\[\ncornbread: \\; goldfinch \\mapsto \\left(1,0,\\frac{1}{3}\\right)+goldfinch(3,6,-2)\n\\]\nand\n\\[\nmarshmallow: \\; starfruit \\mapsto \\left(1,0,-\\frac{1}{2}\\right)+starfruit(-2,6,3).\n\\]\nThese lines cross the given lines (in order) for\n\\[\ngoldfinch = 0,\\; +\\frac{1}{6},\\; -\\frac{1}{3},\\; \\frac{1}{3}\\quad\\text{and}\\quad starfruit = 0,\\; \\frac{1}{6},\\; \\frac{1}{2},\\; \\frac{1}{8} .\n\\]\nIn non-parametric form \\( cornbread \\) is given by\n\\[\nbluegrass = 2(peppermint-1) = 1 - 3\\,driftwood\n\\]\nand \\( marshmallow \\) is given by\n\\[\nbluegrass = 3(1-peppermint) = 2\\,driftwood + 1\n\\]\nSecond Solution. Denote the given lines in order by \\( tortoise, sailboat, snowflake, \\) and \\( pinecones \\). Then the equation of the plane of the required line \\( moonstone \\) and \\( tortoise \\) has the form\n\\[\nbluegrass = jackrabbit(peppermint-1)\n\\]\nThe equation of the plane of \\( moonstone \\) and \\( sailboat \\) has the form\n\\[\ndriftwood = campfires(bluegrass-1)\n\\]\nand the plane of \\( moonstone \\) and \\( snowflake \\) is given by\n\\[\npeppermint = dragonfly(driftwood-1).\n\\]\nAny two of these equations determine the line \\( moonstone \\), so, if we eliminate \\( bluegrass \\) from the first two of these equations, we must obtain an equation equivalent to the third. Therefore \\( jackrabbit\\,campfires\\,dragonfly = 1 \\).\n\nLet the point of intersection of \\( moonstone \\) and \\( pinecones \\) be \\( (6 horseshoe, 6 horseshoe, -horseshoe) \\). It lies on all of the planes considered above, so\n\\[\n\\begin{aligned}\n6\\,horseshoe &= jackrabbit(6\\,horseshoe-1) \\\\\n-horseshoe &= campfires(6\\,horseshoe-1) \\\\\n6\\,horseshoe &= dragonfly(-horseshoe-1).\n\\end{aligned}\n\\]\nMultiply these equations and use \\( jackrabbit\\,campfires\\,dragonfly = 1 \\) to get\n\\[\n-36\\,horseshoe^{3} = -(6\\,horseshoe-1)^{2}(horseshoe+1).\n\\]\nThis simplifies to \\( 24\\,horseshoe^{2} - 11\\,horseshoe + 1 = 0 \\), so \\( horseshoe = \\frac{1}{3} \\) or \\( \\frac{1}{8} \\). The corresponding values of \\( jackrabbit, campfires, dragonfly \\) are \\( 2, -\\frac{1}{3}, -\\frac{3}{2} \\) or \\( -3, +\\frac{1}{2}, -\\frac{2}{3} \\), and we obtain non-parametric equations for \\( cornbread \\) and \\( marshmallow \\) as before.\n\nFor a general treatment of this problem, see D. M. Y. Sommerville, Analytic Geometry of Three Dimensions, Cambridge, 1934, page 184."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "disarrayed",
        "y": "confusion",
        "z": "disorder",
        "L": "curvepath",
        "L_1": "crookedone",
        "L_2": "crookedtwo",
        "M_1": "bendedone",
        "M_2": "bendedtwo",
        "M_3": "bendedthree",
        "M_4": "bendedfour",
        "A": "broadarea",
        "B": "widezone",
        "C": "spacious",
        "D": "expansive",
        "a": "certainly",
        "b": "surevalue",
        "c": "fixedness",
        "d": "setnumber",
        "s": "endresult",
        "t": "solution",
        "\\lambda": "steadfast",
        "\\mu": "firmament",
        "\\nu": "immutable"
      },
      "question": "4. Find the equations of the two straight lines each of which cuts all the four straight lines\n\\[\ndisarrayed=1, confusion=0 ; \\quad confusion=1, disorder=0 ; \\quad disorder=1, disarrayed=0 ; \\quad disarrayed=confusion=-6 disorder\n\\]",
      "solution": "First Solution. Suppose the required line \\( curvepath \\) meets the given lines in the points \\( broadarea, widezone, spacious, \\) and \\( expansive \\) respectively. Then\n\\[\nbroadarea=(1,0, certainly), widezone=(surevalue, 1,0), spacious=(0, fixedness, 1), \\text { and } expansive=(6 setnumber, 6 setnumber,-setnumber)\n\\]\nfor some numbers \\( certainly, surevalue, fixedness, \\) and \\( setnumber \\). Treat \\( broadarea, widezone, spacious, \\) and \\( expansive \\) as vectors. The condition that they be collinear is that the vectors\n\\[\n\\begin{array}{l}\nwidezone-broadarea=(surevalue-1,1,-certainly) \\\\\nspacious-broadarea=(-1, fixedness, 1-certainly) \\\\\nexpansive-broadarea=(6 setnumber-1,6 setnumber,-setnumber-certainly)\n\\end{array}\n\\]\nbe proportional.\nThe proportionality of the first two tells us that\n\\[\nfixedness=\\frac{1}{1-surevalue}=\\frac{certainly-1}{certainly}\n\\]\nwhile the first and third give\n\\[\n6 setnumber=\\frac{1-6 setnumber}{1-surevalue}=\\frac{certainly+setnumber}{certainly}\n\\]\n\nRewrite the middle member here using (1)\n\\[\n6 setnumber=(1-6 setnumber) \\frac{certainly-1}{certainly}=\\frac{certainly+setnumber}{certainly} .\n\\]\n\nClearing fractions\n\\[\n\\begin{aligned}\n6 certainly setnumber & =certainly+setnumber \\\\\ncertainly+6 setnumber-1-6 certainly setnumber & =certainly+setnumber\n\\end{aligned}\n\\]\n\nAdding these equations, we find \\( 4 setnumber=certainly+1 \\), so\n\\[\n6 certainly(certainly+1)=24 certainly setnumber=4(certainly+setnumber)=5 certainly+1\n\\]\n\nThe quadratic equation \\( 6 certainly(certainly+1)=5 certainly+1 \\) has roots\n\\[\ncertainly=\\frac{1}{3},-\\frac{1}{2}\n\\]\nand the corresponding values of the other unknowns are\n\\[\n\\begin{aligned}\nsurevalue & =\\frac{3}{2}, \\frac{2}{3} \\\\\nfixedness & =-2,3 \\\\\nsetnumber & =\\frac{1}{3}, \\frac{1}{8}\n\\end{aligned}\n\\]\n\nThe direction vectors of the lines (proportional to \\( widezone-broadarea, spacious-broadarea, \\) and \\( expansive-broadarea) \\) in the two cases are \\( (3,6,-2) \\) and \\( (-2,6,3) \\), respectively. The two lines are given parametrically by\n\\[\ncrookedone: endresult \\mapsto\\left(1,0, \\frac{1}{3}\\right)+endresult(3,6,-2)\n\\]\nand\n\\[\ncrookedtwo: solution \\mapsto\\left(1,0,-\\frac{1}{2}\\right)+solution(-2,6,3) .\n\\]\n\nThese lines cross the given lines (in order) for\n\\[\nendresult=0,+\\frac{1}{6},-\\frac{1}{3}, \\frac{1}{3} \\text { and } solution=0, \\frac{1}{6}, \\frac{1}{2}, \\frac{1}{8} .\n\\]\n\nIn non-parametric form \\( crookedone \\) is given by\n\\[\nconfusion=2(disarrayed-1)=1-3 disorder\n\\]\nand \\( crookedtwo \\) is given by\n\\[\nconfusion=3(1-disarrayed)=2 disorder+1\n\\]\n\nSecond Solution. Denote the given lines in order by \\( bendedone, bendedtwo, bendedthree, \\) and \\( bendedfour \\). Then the equation of the plane of the required line \\( curvepath \\) and \\( bendedone \\) has the form\n\\[\nconfusion=steadfast(disarrayed-1)\n\\]\n\nThe equation of the plane of \\( curvepath \\) and \\( bendedtwo \\) has the form\n\\[\ndisorder=firmament(confusion-1)\n\\]\nand the plane of \\( curvepath \\) and \\( bendedthree \\) is given by\n\\[\ndisarrayed=immutable(disorder-1) .\n\\]\n\nAny two of these equations determine the line \\( curvepath \\), so, if we eliminate \\( confusion \\) from the first two of these equations, we must obtain an equation equivalent to the third. Therefore \\( steadfast firmament immutable=1 \\).\n\nLet the point of intersection of \\( curvepath \\) and \\( bendedfour \\) be \\( (6 setnumber, 6 setnumber,-setnumber) \\). It lies on all of the planes considered above, so\n\\[\n\\begin{aligned}\n6 setnumber & =steadfast(6 setnumber-1) \\\\\n-setnumber & =firmament(6 setnumber-1) \\\\\n6 setnumber & =immutable(-setnumber-1) .\n\\end{aligned}\n\\]\n\nMultiply these equations and use \\( steadfast firmament immutable=1 \\) to get\n\\[\n-36 setnumber^{3}=-(6 setnumber-1)^{2}(setnumber+1) .\n\\]\n\nThis simplifies to \\( 24 setnumber^{2}-11 setnumber+1=0 \\), so \\( setnumber=\\frac{1}{3} \\) or \\( \\frac{1}{8} \\). The corresponding values of \\( steadfast, firmament, immutable \\) are \\( 2,-\\frac{1}{3},-\\frac{3}{2} \\) or \\( -3,+\\frac{1}{2},-\\frac{2}{3} \\), and we obtain nonparametric equations for \\( crookedone \\) and \\( crookedtwo \\) as before.\n\nFor a general treatment of this problem, see D. M. Y. Sommerville, Analytic Geometry of Three Dimensions, Cambridge, 1934, page 184."
    },
    "garbled_string": {
      "map": {
        "x": "hgfrtlae",
        "y": "kcjmdspo",
        "z": "vnbxqwer",
        "L": "qzxwvtnp",
        "L_1": "tpmzquab",
        "L_2": "nxfqplko",
        "M_1": "sdfrujke",
        "M_2": "bcerlqtm",
        "M_3": "yqplmsdv",
        "M_4": "kzptungh",
        "A": "fqlarnsd",
        "B": "wernjdas",
        "C": "opqmsltd",
        "D": "ujkcbnva",
        "a": "lzxoqmnr",
        "b": "pdvfrsua",
        "c": "gqtwhlzx",
        "d": "mskjture",
        "s": "lprkdhwv",
        "t": "hcgplxzb",
        "\\lambda": "zophtwre",
        "\\mu": "fzbclorh",
        "\\nu": "rmpdqsev"
      },
      "question": "4. Find the equations of the two straight lines each of which cuts all the four straight lines\n\\[\nhgfrtlae=1, kcjmdspo=0 ; \\quad kcjmdspo=1, vnbxqwer=0 ; \\quad vnbxqwer=1, hgfrtlae=0 ; \\quad hgfrtlae=kcjmdspo=-6 vnbxqwer\n\\]\n",
      "solution": "First Solution. Suppose the required line \\( qzxwvtnp \\) meets the given lines in the points \\( fqlarnsd, wernjdas, opqmsltd \\), and \\( ujkcbnva \\) respectively. Then\n\\[\nfqlarnsd=(1,0, lzxoqmnr), wernjdas=(pdvfrsua, 1,0), opqmsltd=(0, gqtwhlzx, 1), \\text { and } ujkcbnva=(6 mskjture, 6 mskjture,-mskjture)\n\\]\nfor some numbers \\( lzxoqmnr, pdvfrsua, gqtwhlzx \\), and \\( mskjture \\). Treat \\( fqlarnsd, wernjdas, opqmsltd \\), and \\( ujkcbnva \\) as vectors. The condition that they be collinear is that the vectors\n\\[\n\\begin{array}{l}\nwernjdas-fqlarnsd=(pdvfrsua-1,1,-lzxoqmnr) \\\\\nopqmsltd-fqlarnsd=(-1, gqtwhlzx, 1-lzxoqmnr) \\\\\nujkcbnva-fqlarnsd=(6 mskjture-1,6 mskjture,-mskjture-lzxoqmnr)\n\\end{array}\n\\]\nbe proportional.\nThe proportionality of the first two tells us that\n\\[\ngqtwhlzx=\\frac{1}{1-pdvfrsua}=\\frac{lzxoqmnr-1}{lzxoqmnr}\n\\]\nwhile the first and third give\n\\[\n6 mskjture=\\frac{1-6 mskjture}{1-pdvfrsua}=\\frac{lzxoqmnr+mskjture}{lzxoqmnr}\n\\]\n\nRewrite the middle member here using (1)\n\\[\n6 mskjture=(1-6 mskjture) \\frac{lzxoqmnr-1}{lzxoqmnr}=\\frac{lzxoqmnr+mskjture}{lzxoqmnr} .\n\\]\n\nClearing fractions\n\\[\n\\begin{aligned}\n6 lzxoqmnr mskjture & =lzxoqmnr+mskjture \\\\\nlzxoqmnr+6 mskjture-1-6 lzxoqmnr mskjture & =lzxoqmnr+mskjture\n\\end{aligned}\n\\]\n\nAdding these equations, we find \\( 4 mskjture=lzxoqmnr+1 \\), so\n\\[\n6 lzxoqmnr(lzxoqmnr+1)=24 lzxoqmnr mskjture=4(lzxoqmnr+mskjture)=5 lzxoqmnr+1\n\\]\n\nThe quadratic equation \\( 6 lzxoqmnr(lzxoqmnr+1)=5 lzxoqmnr+1 \\) has roots\n\\[\nlzxoqmnr=\\frac{1}{3},-\\frac{1}{2}\n\\]\nand the corresponding values of the other unknowns are\n\\[\n\\begin{aligned}\npdvfrsua & =\\frac{3}{2}, \\frac{2}{3} \\\\\ngqtwhlzx & =-2,3 \\\\\nmskjture & =\\frac{1}{3}, \\frac{1}{8}\n\\end{aligned}\n\\]\n\nThe direction vectors of the lines (proportional to \\( wernjdas-fqlarnsd, opqmsltd-fqlarnsd \\), and \\( ujkcbnva-fqlarnsd) \\) in the two cases are \\( (3,6,-2) \\) and \\( (-2,6,3) \\), respectively. The two lines are given parametrically by\n\\[\ntpmzquab: lprkdhwv \\mapsto\\left(1,0, \\frac{1}{3}\\right)+lprkdhwv(3,6,-2)\n\\]\nand\n\\[\nnxfqplko: hcgplxzb \\mapsto\\left(1,0,-\\frac{1}{2}\\right)+hcgplxzb(-2,6,3) .\n\\]\n\nThese lines cross the given lines (in order) for\n\\[\nlprkdhwv=0,+\\frac{1}{6},-\\frac{1}{3}, \\frac{1}{3} \\text { and } hcgplxzb=0, \\frac{1}{6}, \\frac{1}{2}, \\frac{1}{8} .\n\\]\n\nIn non-parametric form \\( tpmzquab \\) is given by\n\\[\nkcjmdspo=2(hgfrtlae-1)=1-3 vnbxqwer\n\\]\nand \\( nxfqplko \\) is given by\n\\[\nkcjmdspo=3(1-hgfrtlae)=2 vnbxqwer+1\n\\]\n\nSecond Solution. Denote the given lines in order by \\( sdfrujke, bcerlqtm, yqplmsdv \\), and \\( kzptungh \\). Then the equation of the plane of the required line \\( qzxwvtnp \\) and \\( sdfrujke \\) has the form\n\\[\nkcjmdspo=zophtwre(hgfrtlae-1)\n\\]\n\nThe equation of the plane of \\( qzxwvtnp \\) and \\( bcerlqtm \\) has the form\n\\[\nvnbxqwer=fzbclorh(kcjmdspo-1)\n\\]\nand the plane of \\( qzxwvtnp \\) and \\( yqplmsdv \\) is given by\n\\[\nhgfrtlae=rmpdqsev(vnbxqwer-1) .\n\\]\n\nAny two of these equations determine the line \\( qzxwvtnp \\), so, if we eliminate \\( kcjmdspo \\) from the first two of these equations, we must obtain an equation equivalent to the third. Therefore \\( zophtwre fzbclorh rmpdqsev=1 \\).\n\nLet the point of intersection of \\( qzxwvtnp \\) and \\( kzptungh \\) be \\( (6 mskjture, 6 mskjture,-mskjture) \\). It lies on all of the planes considered above, so\n\\[\n\\begin{aligned}\n6 mskjture & =zophtwre(6 mskjture-1) \\\\\n-mskjture & =fzbclorh(6 mskjture-1) \\\\\n6 mskjture & =rmpdqsev(-mskjture-1) .\n\\end{aligned}\n\\]\n\nMultiply these equations and use \\( zophtwre fzbclorh rmpdqsev=1 \\) to get\n\\[\n-36 mskjture^{3}=-(6 mskjture-1)^{2}(mskjture+1) .\n\\]\n\nThis simplifies to \\( 24 mskjture^{2}-11 mskjture+1=0 \\), so \\( mskjture=\\frac{1}{3} \\) or \\( \\frac{1}{8} \\). The corresponding values of \\( zophtwre, fzbclorh, rmpdqsev \\) are \\( 2,-\\frac{1}{3},-\\frac{3}{2} \\) or \\( -3,+\\frac{1}{2},-\\frac{2}{3} \\), and we obtain nonparametric equations for \\( tpmzquab \\) and \\( nxfqplko \\) as before.\n\nFor a general treatment of this problem, see D. M. Y. Sommerville, Analytic Geometry of Three Dimensions, Cambridge, 1934, page 184."
    },
    "kernel_variant": {
      "question": "In $\\mathbb{R}^{3}$ consider the four mutually skew straight lines  \n\\[\n\\begin{aligned}\nL_{1}&:\\;x=2,\\;y=0\\qquad\\qquad\\qquad\\quad(\\text{all }z\\in\\mathbb{R}),\\\\[2mm]\nL_{2}&:\\;y=2,\\;z=0\\qquad\\qquad\\qquad\\quad(\\text{all }x\\in\\mathbb{R}),\\\\[2mm]\nL_{3}&:\\;z=2,\\;x=0\\qquad\\qquad\\qquad\\quad(\\text{all }y\\in\\mathbb{R}),\\\\[2mm]\nL_{4}&:\\;x=y=-4\\,z\\qquad\\qquad\\qquad\\;\\;\\;(\\text{all }z\\in\\mathbb{R}),\n\\end{aligned}\n\\]\nand the sphere  \n\\[\n\\Sigma:\\;(x-1)^{2}+(y-1)^{2}+(z-1)^{2}=2 \\qquad (\\text{radius } \\sqrt{2}).\n\\]\n\nDetermine – and prove that you have determined all – straight lines $L$ that simultaneously  \n\\begin{itemize}\n\\item[(i)] intersect each of the four lines $L_{1},\\,L_{2},\\,L_{3}$ and $L_{4}$,  \n\\item[(ii)] are tangent to $\\Sigma$, and  \n\\item[(iii)] touch $\\Sigma$ at a point whose $z$–coordinate is positive (i.e. the point of tangency lies in the half–space $z>0$).  \n\\end{itemize}\n\nFor every such line give both a parametric description $\\;{\\bf r}={\\bf p}+t{\\bf d}$ and an equivalent system of two Cartesian linear equations.",
      "solution": "Let ${\\bf C}=(1,1,1)$ and $R=\\sqrt{2}$ be the centre and the radius of $\\Sigma$.\n\n\\bigskip\n\\textbf{1.\\; All common transversals of the four given lines}\n\nLet the sought transversal $L$ meet $L_{1},\\dots ,L_{4}$ respectively in  \n\\[\nA=(2,0,a),\\quad B=(b,2,0),\\quad C^{\\prime}=(0,c,2),\\quad D=(-4d,-4d,d)\\qquad(a,b,c,d\\in\\mathbb{R}).\\tag{1}\n\\]\n\nBecause $A,B,C^{\\prime},D$ are collinear, the vectors\n\\[\n{\\bf v}:=B-A,\\qquad C^{\\prime}-A=\\lambda{\\bf v},\\qquad D-A=\\mu{\\bf v}\\qquad(\\lambda,\\mu\\in\\mathbb{R})\\tag{2}\n\\]\nare pairwise proportional.  Writing (2) component–wise yields\n\\[\n\\begin{cases}\n-2=\\lambda(b-2),\\\\\n\\;c=2\\lambda,\\\\\n\\;2-a=-\\lambda a,\n\\end{cases}\\qquad\n\\begin{cases}\n-4d-2=\\mu(b-2),\\\\\n-4d=2\\mu,\\\\\n\\;d-a=-\\mu a.\n\\end{cases}\\tag{3}\n\\]\n\nSolving (3) one obtains\n\\[\na=\\dfrac{d}{1+2d},\\qquad\nb=2+\\dfrac{2d+1}{d},\\qquad\nc=-\\dfrac{4d}{1+2d},\\tag{4}\n\\]\ntogether with the quadratic restriction\n\\[\n4d^{2}+7d+2=0\n\\;\\;\\Longleftrightarrow\\;\\;\nd=d_{+}:=\\dfrac{-7+\\sqrt{17}}{8}\\quad\\text{or}\\quad\nd=d_{-}:=\\dfrac{-7-\\sqrt{17}}{8}.\\tag{5}\n\\]\n\nHence there are exactly two common transversals\n\\[\nT_{\\pm}:\\;{\\bf r}={\\bf p}_{\\pm}+t{\\bf v}_{\\pm},\\qquad\n{\\bf p}_{\\pm}=(2,0,a_{\\pm}),\\quad\n{\\bf v}_{\\pm}=(b_{\\pm}-2,\\,2,\\,-a_{\\pm}),\\tag{6}\n\\]\nwhere $(a_{\\pm},b_{\\pm})$ are given by (4) with $d=d_{\\pm}$.\n\n\\bigskip\n\\textbf{2.\\; Tangency to the sphere}\n\nFor a line ${\\bf r}(t)={\\bf p}+t{\\bf v}$ set ${\\bf u}:={\\bf p}-{\\bf C}$.  \nPoints on the line satisfy\n\\[\n\\lVert{\\bf u}+t{\\bf v}\\rVert^{2}=R^{2}=2\n\\;\\;\\Longrightarrow\\;\\;\n|{\\bf v}|^{2}t^{2}+2({\\bf u}\\!\\cdot\\!{\\bf v})t+\\bigl(|{\\bf u}|^{2}-2\\bigr)=0.\\tag{7}\n\\]\nPut $A:=|{\\bf v}|^{2},\\;B:={\\bf u}\\!\\cdot\\!{\\bf v},\\;\\widehat{C}:=|{\\bf u}|^{2}-2$.  \nTangency happens iff the discriminant vanishes:\n\\[\n\\Delta:=B^{2}-A\\widehat{C}=0.\\tag{8}\n\\]\n\nInserting the parametrisations (4)–(6) and eliminating gives\n\\[\n\\Delta(d)=\\dfrac{(4d^{2}+7d+2)(d^{2}+3d+1)^{2}}{d^{2}(1+2d)^{4}}.\\tag{9}\n\\]\nThus $\\Delta(d_{\\pm})=0$ because $4d^{2}+7d+2=0$ for $d=d_{\\pm}$; so \\emph{both} $T_{+}$ and $T_{-}$ are tangent to $\\Sigma$.\n\n\\bigskip\n\\textbf{3.\\; The half–space condition $z>0$}\n\nFor a tangent line, the (unique) point of contact $P$ occurs at\n\\[\nt_{0}=-\\dfrac{B}{A},\\qquad P={\\bf p}+t_{0}{\\bf v}.\\tag{10}\n\\]\n\nAlong either transversal $z(t)=a+t(-a)=a(1-t)$, hence\n\\[\nP_{z}=a(1-t_{0})=a\\,\\dfrac{A+B}{A}.\\tag{11}\n\\]\nBecause $A=|{\\bf v}|^{2}>0$, $\\operatorname{sign}(P_{z})=\\operatorname{sign}\\bigl(a(A+B)\\bigr)$.  \nUsing (4) one finds\n\\[\nA+B=\\frac{8+\\dfrac{5}{d}+\\dfrac{1}{d^{2}}+\\dfrac{d}{1+2d}}{1}\n       =\\frac{N(d)}{d^{2}(1+2d)},\\qquad\nN(d)=17d^{3}+18d^{2}+7d+1.\\tag{12}\n\\]\n\nEmploying $4d^{2}+7d+2=0$ to eliminate $d^{2}$ and $d^{3}$ gives\n\\[\nN(d)=-\\frac{47}{4}d^{2}-\\frac{3}{2}d+1\n     =\\frac{110+305d}{16}.\\tag{13}\n\\]\n\nHence\n\\[\n\\operatorname{sign}(A+B)=\n\\operatorname{sign}(110+305d)\\,\\cdot\\,\n\\operatorname{sign}(1+2d).\\tag{14}\n\\]\n\nAt the two admissible values,\n\\[\n\\begin{aligned}\nd_{+}:&\\quad 110+305d_{+}=\\dfrac{-1255+305\\sqrt{17}}{8}>0,\\qquad 1+2d_{+}>0;\\\\[1mm]\nd_{-}:&\\quad 110+305d_{-}=\\dfrac{-1255-305\\sqrt{17}}{8}<0,\\qquad 1+2d_{-}<0.\n\\end{aligned}\n\\]\nThus in \\emph{both} cases $A+B>0$, so $\\operatorname{sign}(P_{z})=\\operatorname{sign}(a)$.  \nBecause $a=d/(1+2d)$, formula (4) gives\n\\[\na_{+}<0\\quad\\Longrightarrow\\quad(P_{+})_{z}<0,\\qquad\na_{-}>0\\quad\\Longrightarrow\\quad(P_{-})_{z}>0.\\tag{15}\n\\]\n\nTherefore exactly one of the two tangent transversals, namely $T_{-}$, meets $\\Sigma$ at a point with $z>0$.\n\n\\bigskip\n\\textbf{4.\\; Uniqueness}\n\nSection 1 shows that there are no additional common transversals; Sections 2–3 show that only $T_{-}$ fulfils condition (iii).  Hence the required line is unique.\n\n\\bigskip\n\\textbf{5.\\; Explicit equations of the unique line}\n\nPut $\\Delta:=\\sqrt{17}$ and $d:=d_{-}=(-7-\\Delta)/8$.\n\n\\begin{itemize}\n\\item[(i)] Point on the line (its intersection with $L_{1}$):\n\\[\na=\\frac{d}{1+2d}=\\frac{7+\\Delta}{2(3+\\Delta)},\\qquad\n{\\bf p}=(2,0,a).\\tag{16}\n\\]\n\n\\item[(ii)] Direction vector:\n\\[\n{\\bf v}=(b-2,\\,2,\\,-a)=\n\\Bigl(\\,\\frac{2(3+\\Delta)}{7+\\Delta},\\;2,\\;-\\frac{7+\\Delta}{2(3+\\Delta)}\\Bigr).\\tag{17}\n\\]\n\n\\item[(iii)] Parametric form ($t\\in\\mathbb{R}$):\n\\[\nL:\\;\n(x,y,z)=\\Bigl(2,\\,0,\\,\\frac{7+\\Delta}{2(3+\\Delta)}\\Bigr)+\nt\\Bigl(\\frac{2(3+\\Delta)}{7+\\Delta},\\,2,\\,-\\frac{7+\\Delta}{2(3+\\Delta)}\\Bigr).\\tag{18}\n\\]\n\n\\item[(iv)] Cartesian form.  \nFrom the $y$–coordinate, $t=y/2$.  \nSubstituting this into the $x$– and $z$–coordinates of (18) yields the two independent linear equations\n\\[\n\\boxed{\\;\nx-2=\\frac{3+\\Delta}{7+\\Delta}\\,y,\\qquad\n(7+\\Delta)y+4(3+\\Delta)z=2(7+\\Delta)\\;}. \\tag{19}\n\\]\n\\end{itemize}\n\n\\bigskip\n\\textbf{Answer.}  \nExactly one straight line satisfies all three requirements; it is given by the parametric equations (18) or, equivalently, by the Cartesian system (19).\n\n\\bigskip",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.350254",
        "was_fixed": false,
        "difficulty_analysis": "1. Additional geometric constraint.  Besides intersecting the four skew lines, the sought line must also be tangent to a given sphere.  This introduces a quadratic (distance) condition, forcing the solver to blend linear‐algebraic reasoning with differential (normal-vector) considerations.\n\n2. Filtering by an angle inequality.  Even after satisfying the tangency condition, one must still test an angular sign constraint, eliminating spurious solutions that earlier stages leave unobstructed.\n\n3. Layered decision process.  The solver must  \n • find all common transversals (a full solution of the original problem),  \n • compute distances from these transversals to a point,  \n • verify a quadratic tangency equation, and  \n • analyse angular data.  \nEach layer may discard previously legitimate candidates, so any omission or mis-calculation anywhere prevents success.\n\n4. Computational heft.  The tangency test requires non-trivial vector products and norm calculations; the acute-angle test uses dot products and careful sign control.\n\n5. Conceptual breadth.  The problem mixes projective transversal theory, Euclidean distance/tangency, and directional (angle) geometry, compelling the contestant to command several distinct areas of three-dimensional analytic geometry in one coherent argument.\n\nAll these features markedly intensify the original exercise, which terminated once the two transversals had been located."
      }
    },
    "original_kernel_variant": {
      "question": "In $\\mathbb{R}^{3}$ consider the four mutually skew straight lines  \n\\[\n\\begin{aligned}\nL_{1}&:\\;x=2,\\;y=0\\qquad\\qquad\\qquad\\quad(\\text{all }z\\in\\mathbb{R}),\\\\[2mm]\nL_{2}&:\\;y=2,\\;z=0\\qquad\\qquad\\qquad\\quad(\\text{all }x\\in\\mathbb{R}),\\\\[2mm]\nL_{3}&:\\;z=2,\\;x=0\\qquad\\qquad\\qquad\\quad(\\text{all }y\\in\\mathbb{R}),\\\\[2mm]\nL_{4}&:\\;x=y=-4\\,z\\qquad\\qquad\\qquad\\;\\;\\;(\\text{all }z\\in\\mathbb{R}),\n\\end{aligned}\n\\]\nand the sphere  \n\\[\n\\Sigma:\\;(x-1)^{2}+(y-1)^{2}+(z-1)^{2}=2 \\qquad (\\text{radius } \\sqrt{2}).\n\\]\n\nDetermine – and prove that you have determined all – straight lines $L$ that simultaneously  \n\\begin{itemize}\n\\item[(i)] intersect each of the four lines $L_{1},\\,L_{2},\\,L_{3}$ and $L_{4}$,  \n\\item[(ii)] are tangent to $\\Sigma$, and  \n\\item[(iii)] touch $\\Sigma$ at a point whose $z$–coordinate is positive (i.e. the point of tangency lies in the half–space $z>0$).  \n\\end{itemize}\n\nFor every such line give both a parametric description $\\;{\\bf r}={\\bf p}+t{\\bf d}$ and an equivalent system of two Cartesian linear equations.",
      "solution": "Let ${\\bf C}=(1,1,1)$ and $R=\\sqrt{2}$ be the centre and the radius of $\\Sigma$.\n\n\\bigskip\n\\textbf{1.\\; All common transversals of the four given lines}\n\nLet the sought transversal $L$ meet $L_{1},\\dots ,L_{4}$ respectively in  \n\\[\nA=(2,0,a),\\quad B=(b,2,0),\\quad C^{\\prime}=(0,c,2),\\quad D=(-4d,-4d,d)\\qquad(a,b,c,d\\in\\mathbb{R}).\\tag{1}\n\\]\n\nBecause $A,B,C^{\\prime},D$ are collinear, the vectors\n\\[\n{\\bf v}:=B-A,\\qquad C^{\\prime}-A=\\lambda{\\bf v},\\qquad D-A=\\mu{\\bf v}\\qquad(\\lambda,\\mu\\in\\mathbb{R})\\tag{2}\n\\]\nare pairwise proportional.  Writing (2) component–wise yields\n\\[\n\\begin{cases}\n-2=\\lambda(b-2),\\\\\n\\;c=2\\lambda,\\\\\n\\;2-a=-\\lambda a,\n\\end{cases}\\qquad\n\\begin{cases}\n-4d-2=\\mu(b-2),\\\\\n-4d=2\\mu,\\\\\n\\;d-a=-\\mu a.\n\\end{cases}\\tag{3}\n\\]\n\nSolving (3) one obtains\n\\[\na=\\dfrac{d}{1+2d},\\qquad\nb=2+\\dfrac{2d+1}{d},\\qquad\nc=-\\dfrac{4d}{1+2d},\\tag{4}\n\\]\ntogether with the quadratic restriction\n\\[\n4d^{2}+7d+2=0\n\\;\\;\\Longleftrightarrow\\;\\;\nd=d_{+}:=\\dfrac{-7+\\sqrt{17}}{8}\\quad\\text{or}\\quad\nd=d_{-}:=\\dfrac{-7-\\sqrt{17}}{8}.\\tag{5}\n\\]\n\nHence there are exactly two common transversals\n\\[\nT_{\\pm}:\\;{\\bf r}={\\bf p}_{\\pm}+t{\\bf v}_{\\pm},\\qquad\n{\\bf p}_{\\pm}=(2,0,a_{\\pm}),\\quad\n{\\bf v}_{\\pm}=(b_{\\pm}-2,\\,2,\\,-a_{\\pm}),\\tag{6}\n\\]\nwhere $(a_{\\pm},b_{\\pm})$ are given by (4) with $d=d_{\\pm}$.\n\n\\bigskip\n\\textbf{2.\\; Tangency to the sphere}\n\nFor a line ${\\bf r}(t)={\\bf p}+t{\\bf v}$ set ${\\bf u}:={\\bf p}-{\\bf C}$.  \nPoints on the line satisfy\n\\[\n\\lVert{\\bf u}+t{\\bf v}\\rVert^{2}=R^{2}=2\n\\;\\;\\Longrightarrow\\;\\;\n|{\\bf v}|^{2}t^{2}+2({\\bf u}\\!\\cdot\\!{\\bf v})t+\\bigl(|{\\bf u}|^{2}-2\\bigr)=0.\\tag{7}\n\\]\nPut $A:=|{\\bf v}|^{2},\\;B:={\\bf u}\\!\\cdot\\!{\\bf v},\\;\\widehat{C}:=|{\\bf u}|^{2}-2$.  \nTangency happens iff the discriminant vanishes:\n\\[\n\\Delta:=B^{2}-A\\widehat{C}=0.\\tag{8}\n\\]\n\nInserting the parametrisations (4)–(6) and eliminating gives\n\\[\n\\Delta(d)=\\dfrac{(4d^{2}+7d+2)(d^{2}+3d+1)^{2}}{d^{2}(1+2d)^{4}}.\\tag{9}\n\\]\nThus $\\Delta(d_{\\pm})=0$ because $4d^{2}+7d+2=0$ for $d=d_{\\pm}$; so \\emph{both} $T_{+}$ and $T_{-}$ are tangent to $\\Sigma$.\n\n\\bigskip\n\\textbf{3.\\; The half–space condition $z>0$}\n\nFor a tangent line, the (unique) point of contact $P$ occurs at\n\\[\nt_{0}=-\\dfrac{B}{A},\\qquad P={\\bf p}+t_{0}{\\bf v}.\\tag{10}\n\\]\n\nAlong either transversal $z(t)=a+t(-a)=a(1-t)$, hence\n\\[\nP_{z}=a(1-t_{0})=a\\,\\dfrac{A+B}{A}.\\tag{11}\n\\]\nBecause $A=|{\\bf v}|^{2}>0$, $\\operatorname{sign}(P_{z})=\\operatorname{sign}\\bigl(a(A+B)\\bigr)$.  \nUsing (4) one finds\n\\[\nA+B=\\frac{8+\\dfrac{5}{d}+\\dfrac{1}{d^{2}}+\\dfrac{d}{1+2d}}{1}\n       =\\frac{N(d)}{d^{2}(1+2d)},\\qquad\nN(d)=17d^{3}+18d^{2}+7d+1.\\tag{12}\n\\]\n\nEmploying $4d^{2}+7d+2=0$ to eliminate $d^{2}$ and $d^{3}$ gives\n\\[\nN(d)=-\\frac{47}{4}d^{2}-\\frac{3}{2}d+1\n     =\\frac{110+305d}{16}.\\tag{13}\n\\]\n\nHence\n\\[\n\\operatorname{sign}(A+B)=\n\\operatorname{sign}(110+305d)\\,\\cdot\\,\n\\operatorname{sign}(1+2d).\\tag{14}\n\\]\n\nAt the two admissible values,\n\\[\n\\begin{aligned}\nd_{+}:&\\quad 110+305d_{+}=\\dfrac{-1255+305\\sqrt{17}}{8}>0,\\qquad 1+2d_{+}>0;\\\\[1mm]\nd_{-}:&\\quad 110+305d_{-}=\\dfrac{-1255-305\\sqrt{17}}{8}<0,\\qquad 1+2d_{-}<0.\n\\end{aligned}\n\\]\nThus in \\emph{both} cases $A+B>0$, so $\\operatorname{sign}(P_{z})=\\operatorname{sign}(a)$.  \nBecause $a=d/(1+2d)$, formula (4) gives\n\\[\na_{+}<0\\quad\\Longrightarrow\\quad(P_{+})_{z}<0,\\qquad\na_{-}>0\\quad\\Longrightarrow\\quad(P_{-})_{z}>0.\\tag{15}\n\\]\n\nTherefore exactly one of the two tangent transversals, namely $T_{-}$, meets $\\Sigma$ at a point with $z>0$.\n\n\\bigskip\n\\textbf{4.\\; Uniqueness}\n\nSection 1 shows that there are no additional common transversals; Sections 2–3 show that only $T_{-}$ fulfils condition (iii).  Hence the required line is unique.\n\n\\bigskip\n\\textbf{5.\\; Explicit equations of the unique line}\n\nPut $\\Delta:=\\sqrt{17}$ and $d:=d_{-}=(-7-\\Delta)/8$.\n\n\\begin{itemize}\n\\item[(i)] Point on the line (its intersection with $L_{1}$):\n\\[\na=\\frac{d}{1+2d}=\\frac{7+\\Delta}{2(3+\\Delta)},\\qquad\n{\\bf p}=(2,0,a).\\tag{16}\n\\]\n\n\\item[(ii)] Direction vector:\n\\[\n{\\bf v}=(b-2,\\,2,\\,-a)=\n\\Bigl(\\,\\frac{2(3+\\Delta)}{7+\\Delta},\\;2,\\;-\\frac{7+\\Delta}{2(3+\\Delta)}\\Bigr).\\tag{17}\n\\]\n\n\\item[(iii)] Parametric form ($t\\in\\mathbb{R}$):\n\\[\nL:\\;\n(x,y,z)=\\Bigl(2,\\,0,\\,\\frac{7+\\Delta}{2(3+\\Delta)}\\Bigr)+\nt\\Bigl(\\frac{2(3+\\Delta)}{7+\\Delta},\\,2,\\,-\\frac{7+\\Delta}{2(3+\\Delta)}\\Bigr).\\tag{18}\n\\]\n\n\\item[(iv)] Cartesian form.  \nFrom the $y$–coordinate, $t=y/2$.  \nSubstituting this into the $x$– and $z$–coordinates of (18) yields the two independent linear equations\n\\[\n\\boxed{\\;\nx-2=\\frac{3+\\Delta}{7+\\Delta}\\,y,\\qquad\n(7+\\Delta)y+4(3+\\Delta)z=2(7+\\Delta)\\;}. \\tag{19}\n\\]\n\\end{itemize}\n\n\\bigskip\n\\textbf{Answer.}  \nExactly one straight line satisfies all three requirements; it is given by the parametric equations (18) or, equivalently, by the Cartesian system (19).\n\n\\bigskip",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.305245",
        "was_fixed": false,
        "difficulty_analysis": "1. Additional geometric constraint.  Besides intersecting the four skew lines, the sought line must also be tangent to a given sphere.  This introduces a quadratic (distance) condition, forcing the solver to blend linear‐algebraic reasoning with differential (normal-vector) considerations.\n\n2. Filtering by an angle inequality.  Even after satisfying the tangency condition, one must still test an angular sign constraint, eliminating spurious solutions that earlier stages leave unobstructed.\n\n3. Layered decision process.  The solver must  \n • find all common transversals (a full solution of the original problem),  \n • compute distances from these transversals to a point,  \n • verify a quadratic tangency equation, and  \n • analyse angular data.  \nEach layer may discard previously legitimate candidates, so any omission or mis-calculation anywhere prevents success.\n\n4. Computational heft.  The tangency test requires non-trivial vector products and norm calculations; the acute-angle test uses dot products and careful sign control.\n\n5. Conceptual breadth.  The problem mixes projective transversal theory, Euclidean distance/tangency, and directional (angle) geometry, compelling the contestant to command several distinct areas of three-dimensional analytic geometry in one coherent argument.\n\nAll these features markedly intensify the original exercise, which terminated once the two transversals had been located."
      }
    }
  },
  "checked": true,
  "problem_type": "calculation",
  "iteratively_fixed": true
}