{
  "index": "1951-A-6",
  "type": "GEO",
  "tag": [
    "GEO",
    "ANA"
  ],
  "difficulty": "",
  "question": "6. Determine the position of a normal chord of a parabola such that it cuts off of the parabola a segment of minimum area.",
  "solution": "Solution. Choose coordinates so that the equation of the parabola is 4ay \\( =x^{2}, a>0 \\).\n\nThe chord connecting the point \\( P\\left(2 a s, a s^{2}\\right) \\) to the point \\( Q\\left(2 a t, a t^{2}\\right) \\) has the equation\n\\[\ny=\\frac{1}{2}(t+s) x-a s t\n\\]\nand the tangent line at (2at, at \\( { }^{2} \\) ) has slope \\( t \\). Hence the line (1) will be normal to the parabola at \\( Q \\) if and only if\n\\[\n\\frac{1}{2} t(t+s)=-1\n\\]\nwhich may be written as\n\\[\ns=-\\frac{2}{t}-t\n\\]\n\nWe see, therefore, that \\( s \\) and \\( t \\) have opposite signs. Take \\( s<0 \\) and \\( t>0 \\). Then the area cut off by the chord is\n\\[\n\\int_{2 a s}^{2 a t}\\left[\\frac{1}{2}(t+s) x-a s t-\\frac{1}{4 a} x^{2}\\right] d x=\\frac{1}{3} a^{2}(t-s)^{3}\n\\]\n\nThis area will be minimal when \\( t-s \\) is minimal. But\n\\[\nt-s=2 t+\\frac{2}{t}=2\\left(\\sqrt{t}-\\frac{1}{\\sqrt{t}}\\right)^{2}+4 \\geq 4\n\\]\n\nEquality is attained only when \\( \\sqrt{t}=1 \\) and hence \\( t=1 \\).\nThus, of all normals to the parabola at points to the right of the axis the normal at \\( (2 a, a) \\) cuts off the least area. The area cut off is \\( 64 a^{2} / 3 \\).\n\nBy symmetry, the normal at \\( (-2 a, a) \\) cuts off the least area among normals at points to the left of the axis.\n\nThe critical normals can be characterized as those which meet the axis at an angle of \\( 45^{\\circ} \\).",
  "vars": [
    "x",
    "y",
    "s",
    "t",
    "P",
    "Q"
  ],
  "params": [
    "a"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "horizvar",
        "y": "vertivar",
        "s": "lambdaval",
        "t": "muvalue",
        "P": "pointone",
        "Q": "pointtwo",
        "a": "parscale"
      },
      "question": "6. Determine the position of a normal chord of a parabola such that it cuts off of the parabola a segment of minimum area.",
      "solution": "Solution. Choose coordinates so that the equation of the parabola is \\(4\\parscale\\vertivar=\\horizvar^{2},\\;\\parscale>0\\).\\n\\nThe chord connecting the point \\(\\pointone\\left(2\\parscale\\lambdaval,\\;\\parscale\\lambdaval^{2}\\right)\\) to the point \\(\\pointtwo\\left(2\\parscale\\muvalue,\\;\\parscale\\muvalue^{2}\\right)\\) has the equation\\n\\[\\n\\vertivar=\\frac{1}{2}(\\muvalue+\\lambdaval)\\horizvar-\\parscale\\lambdaval\\muvalue\\n\\]and the tangent line at \\((2\\parscale\\muvalue,\\;\\parscale\\muvalue^{2})\\) has slope \\(\\muvalue\\). Hence the line (1) will be normal to the parabola at \\(\\pointtwo\\) if and only if\\n\\[\\n\\frac{1}{2}\\muvalue(\\muvalue+\\lambdaval)=-1,\\n\\]which may be written as\\n\\[\\n\\lambdaval=-\\frac{2}{\\muvalue}-\\muvalue.\\n\\]\\n\\nWe see, therefore, that \\(\\lambdaval\\) and \\(\\muvalue\\) have opposite signs. Take \\(\\lambdaval<0\\) and \\(\\muvalue>0\\). Then the area cut off by the chord is\\n\\[\\n\\int_{2\\parscale\\lambdaval}^{2\\parscale\\muvalue}\\left[\\frac{1}{2}(\\muvalue+\\lambdaval)\\horizvar-\\parscale\\lambdaval\\muvalue-\\frac{1}{4\\parscale}\\horizvar^{2}\\right]d\\horizvar=\\frac{1}{3}\\parscale^{2}(\\muvalue-\\lambdaval)^{3}.\\n\\]\\n\\nThis area will be minimal when \\(\\muvalue-\\lambdaval\\) is minimal. But\\n\\[\\n\\muvalue-\\lambdaval=2\\muvalue+\\frac{2}{\\muvalue}=2\\left(\\sqrt{\\muvalue}-\\frac{1}{\\sqrt{\\muvalue}}\\right)^{2}+4\\ge 4.\\n\\]\\n\\nEquality is attained only when \\(\\sqrt{\\muvalue}=1\\) and hence \\(\\muvalue=1\\).\\nThus, of all normals to the parabola at points to the right of the axis, the normal at \\((2\\parscale,\\;\\parscale)\\) cuts off the least area. The area cut off is \\(64\\parscale^{2}/3\\).\\n\\nBy symmetry, the normal at \\((-2\\parscale,\\;\\parscale)\\) cuts off the least area among normals at points to the left of the axis.\\n\\nThe critical normals can be characterized as those which meet the axis at an angle of \\(45^{\\circ}\\)."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "lighthouse",
        "y": "horsepower",
        "s": "backpack",
        "t": "stoneware",
        "P": "waterfall",
        "Q": "pineapple",
        "a": "moonlight"
      },
      "question": "6. Determine the position of a normal chord of a parabola such that it cuts off of the parabola a segment of minimum area.",
      "solution": "Solution. Choose coordinates so that the equation of the parabola is 4 moonlight horsepower = lighthouse^{2}, moonlight>0.\n\nThe chord connecting the point \\( waterfall\\left(2 moonlight backpack, moonlight backpack^{2}\\right) \\) to the point \\( pineapple\\left(2 moonlight stoneware, moonlight stoneware^{2}\\right) \\) has the equation\n\\[\nhorsepower=\\frac{1}{2}(stoneware+backpack) lighthouse-moonlight backpack stoneware\n\\]\nand the tangent line at (2 moonlight stoneware, moonlight stoneware^{2}) has slope \\( stoneware \\). Hence the line (1) will be normal to the parabola at \\( pineapple \\) if and only if\n\\[\n\\frac{1}{2} stoneware(stoneware+backpack)=-1\n\\]\nwhich may be written as\n\\[\nbackpack=-\\frac{2}{stoneware}-stoneware\n\\]\n\nWe see, therefore, that \\( backpack \\) and \\( stoneware \\) have opposite signs. Take \\( backpack<0 \\) and \\( stoneware>0 \\). Then the area cut off by the chord is\n\\[\n\\int_{2 moonlight backpack}^{2 moonlight stoneware}\\left[\\frac{1}{2}(stoneware+backpack) lighthouse-moonlight backpack stoneware-\\frac{1}{4 moonlight} lighthouse^{2}\\right] d lighthouse=\\frac{1}{3} moonlight^{2}(stoneware-backpack)^{3}\n\\]\n\nThis area will be minimal when \\( stoneware-backpack \\) is minimal. But\n\\[\nstoneware-backpack=2 stoneware+\\frac{2}{stoneware}=2\\left(\\sqrt{stoneware}-\\frac{1}{\\sqrt{stoneware}}\\right)^{2}+4 \\geq 4\n\\]\n\nEquality is attained only when \\( \\sqrt{stoneware}=1 \\) and hence \\( stoneware=1 \\).\nThus, of all normals to the parabola at points to the right of the axis the normal at \\( (2 moonlight, moonlight) \\) cuts off the least area. The area cut off is \\( 64 moonlight^{2} / 3 \\).\n\nBy symmetry, the normal at \\( (-2 moonlight, moonlight) \\) cuts off the least area among normals at points to the left of the axis.\n\nThe critical normals can be characterized as those which meet the axis at an angle of \\( 45^{\\circ} \\)."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "verticalco",
        "y": "horizontal",
        "s": "constant",
        "t": "spacevalue",
        "P": "linefigure",
        "Q": "areafactor",
        "a": "variable"
      },
      "question": "6. Determine the position of a normal chord of a parabola such that it cuts off of the parabola a segment of minimum area.",
      "solution": "Solution. Choose coordinates so that the equation of the parabola is 4variablehorizontal \\( =verticalco^{2}, variable>0 \\).\n\nThe chord connecting the point \\( linefigure\\left(2 variable constant, variable constant^{2}\\right) \\) to the point \\( areafactor\\left(2 variable spacevalue, variable spacevalue^{2}\\right) \\) has the equation\n\\[\nhorizontal=\\frac{1}{2}(spacevalue+constant) verticalco-variable constant spacevalue\n\\]\nand the tangent line at (2variable spacevalue, variable spacevalue \\( { }^{2} \\) ) has slope \\( spacevalue \\). Hence the line (1) will be normal to the parabola at \\( areafactor \\) if and only if\n\\[\n\\frac{1}{2} spacevalue(spacevalue+constant)=-1\n\\]\nwhich may be written as\n\\[\nconstant=-\\frac{2}{spacevalue}-spacevalue\n\\]\n\nWe see, therefore, that \\( constant \\) and \\( spacevalue \\) have opposite signs. Take \\( constant<0 \\) and \\( spacevalue>0 \\). Then the area cut off by the chord is\n\\[\n\\int_{2 variable constant}^{2 variable spacevalue}\\left[\\frac{1}{2}(spacevalue+constant) verticalco-variable constant spacevalue-\\frac{1}{4 variable} verticalco^{2}\\right] d verticalco=\\frac{1}{3} variable^{2}(spacevalue-constant)^{3}\n\\]\n\nThis area will be minimal when \\( spacevalue-constant \\) is minimal. But\n\\[\nspacevalue-constant=2 spacevalue+\\frac{2}{spacevalue}=2\\left(\\sqrt{spacevalue}-\\frac{1}{\\sqrt{spacevalue}}\\right)^{2}+4 \\geq 4\n\\]\n\nEquality is attained only when \\( \\sqrt{spacevalue}=1 \\) and hence \\( spacevalue=1 \\).\nThus, of all normals to the parabola at points to the right of the axis the normal at \\( (2 variable, variable) \\) cuts off the least area. The area cut off is \\( 64 variable^{2} / 3 \\).\n\nBy symmetry, the normal at \\( (-2 variable, variable) \\) cuts off the least area among normals at points to the left of the axis.\n\nThe critical normals can be characterized as those which meet the axis at an angle of \\( 45^{\\circ} \\)."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "y": "hjgrksla",
        "s": "vldmqhce",
        "t": "bkpnrfta",
        "P": "zydwmrga",
        "Q": "flsctdne",
        "a": "mpthkqrs"
      },
      "question": "6. Determine the position of a normal chord of a parabola such that it cuts off of the parabola a segment of minimum area.",
      "solution": "Solution. Choose coordinates so that the equation of the parabola is 4mpthkqrshjgrksla \\( =qzxwvtnp^{2}, mpthkqrs>0 \\).\n\nThe chord connecting the point \\( zydwmrga\\left(2 mpthkqrs vldmqhce, mpthkqrs vldmqhce^{2}\\right) \\) to the point \\( flsctdne\\left(2 mpthkqrs bkpnrfta, mpthkqrs bkpnrfta^{2}\\right) \\) has the equation\n\\[\nhjgrksla=\\frac{1}{2}(bkpnrfta+vldmqhce) \\, qzxwvtnp- mpthkqrs vldmqhce bkpnrfta\n\\]\nand the tangent line at \\((2 mpthkqrs bkpnrfta, mpthkqrs bkpnrfta^{2})\\) has slope \\( bkpnrfta \\). Hence the line (1) will be normal to the parabola at \\( flsctdne \\) if and only if\n\\[\n\\frac{1}{2} bkpnrfta(bkpnrfta+vldmqhce)=-1\n\\]\nwhich may be written as\n\\[\nvldmqhce=-\\frac{2}{bkpnrfta}-bkpnrfta\n\\]\n\nWe see, therefore, that \\( vldmqhce \\) and \\( bkpnrfta \\) have opposite signs. Take \\( vldmqhce<0 \\) and \\( bkpnrfta>0 \\). Then the area cut off by the chord is\n\\[\n\\int_{2 mpthkqrs vldmqhce}^{2 mpthkqrs bkpnrfta}\\left[\\frac{1}{2}(bkpnrfta+vldmqhce) qzxwvtnp-mpthkqrs vldmqhce bkpnrfta-\\frac{1}{4 mpthkqrs} qzxwvtnp^{2}\\right] d qzxwvtnp=\\frac{1}{3} mpthkqrs^{2}(bkpnrfta-vldmqhce)^{3}\n\\]\n\nThis area will be minimal when \\( bkpnrfta-vldmqhce \\) is minimal. But\n\\[\nbkpnrfta-vldmqhce=2 bkpnrfta+\\frac{2}{bkpnrfta}=2\\left(\\sqrt{bkpnrfta}-\\frac{1}{\\sqrt{bkpnrfta}}\\right)^{2}+4 \\geq 4\n\\]\n\nEquality is attained only when \\( \\sqrt{bkpnrfta}=1 \\) and hence \\( bkpnrfta=1 \\).\nThus, of all normals to the parabola at points to the right of the axis the normal at \\( (2 mpthkqrs, mpthkqrs) \\) cuts off the least area. The area cut off is \\( 64 mpthkqrs^{2} / 3 \\).\n\nBy symmetry, the normal at \\( (-2 mpthkqrs, mpthkqrs) \\) cuts off the least area among normals at points to the left of the axis.\n\nThe critical normals can be characterized as those which meet the axis at an angle of \\( 45^{\\circ} \\)."
    },
    "kernel_variant": {
      "question": "Consider the upward–opening parabola  \n\\[\nx^{2}=4ay , \\qquad a>0 ,\n\\]  \nwhose focus is $F(0,a)$ and whose directrix is the horizontal line $y=-a$.\n\nFor every chord $PQ$ of the parabola the end–point with the smaller $y$–coordinate is denoted by $P$.  \nA chord is called normal if it is perpendicular to the tangent to the parabola at $P$.\n\nLet $\\Sigma(PQ)$ be the finite parabolic segment bounded by the arc $\\widehat{PQ}$ and the chord $PQ$.  \nRotate $\\Sigma(PQ)$ about the directrix $y=-a$ and denote the resulting solid of revolution by $K(P)$.\n\n1.  Among all normal chords $PQ$ determine the position of the point $P$ for which the volume $V(P)$ of $K(P)$ is minimal.  \n2.  Express that minimal volume explicitly in terms of $a$.\n\n(Obviously there will be two symmetric optimal chords, situated on the left and on the right branch of the parabola.  It suffices to find one of them.)  \n\n──────────────────────────────────",
      "solution": "Throughout we work on the right branch of the parabola ($x>0$); by symmetry the reflected results hold for the left branch.\n\n1.  Parametrisation and the normality condition  \n\n\tIntroduce the standard parameter  \n\t\\[\n\t(x,y)=(2au,\\,au^{2}),\\qquad u\\in\\mathbb{R}.\n\t\\tag{0}\n\t\\]  \n\tLet  \n\t\\[\n\tP=P(t)=(2at,\\,at^{2}), \\quad t>0, \\qquad  \n\tQ=Q(s)=(2as,\\,as^{2}), \\quad s<0 .\n\t\\]\n\tThe tangent at $P$ has slope $t$, so the normal through $P$ has slope $-\\dfrac1t$.  \n\tThe slope of the chord $PQ$ equals $\\dfrac12\\,(t+s)$, hence the normality requirement\n\t\\[\n\t\\dfrac12\\,(t+s)=-\\dfrac1t\\quad\\Longrightarrow\\quad\n\ts=-t-\\dfrac{2}{t}.\n\t\\tag{1}\n\t\\]\n\tConsequently every normal chord is uniquely determined by $t>0$.  Frequently used identities are\n\t\\[\n\tt-s =2\\!\\left(t+\\dfrac1t\\right),\\qquad\n\tt+s=-\\dfrac{2}{t},\\qquad\n\tst=-(t^{2}+2).\n\t\\tag{2}\n\t\\]\n\n2.  Area of the generating parabolic segment  \n\n\tThe line $PQ$ is $y=\\dfrac12(t+s)x-ast$.  Direct integration gives\n\t\\[\n\tA(t)\n\t    =\\int_{x_1}^{x_2}\\!\\Bigl[\\bigl(\\tfrac12(t+s)x-ast\\bigr)-\\tfrac{x^{2}}{4a}\\Bigr]\\,{\\rm d}x\n\t    =\\dfrac{a^{2}}{3}\\,(t-s)^{3}>0 .\n\t\\tag{3}\n\t\\]\n\n3.  $y$–coordinate of the centroid of $\\Sigma(PQ)$  \n\n\tDenote the centroid by $G=(\\bar X,\\bar Y)$.  Using vertical slices,\n\t\\[\n\t\\bar Y\\,A(t)=\\dfrac12\\int_{x_1}^{x_2}\\!\n\t               \\Bigl[\\bigl(\\tfrac12(t+s)x-ast\\bigr)^{2}-\\Bigl(\\dfrac{x^{2}}{4a}\\Bigr)^{2}\\Bigr]\n\t             \\,{\\rm d}x .\n\t\\tag{4}\n\t\\]\n\tStraightforward evaluation and insertion of (1)–(2) yields  \n\t\\[\n\t\\bar Y\n\t   =a\\,\\dfrac{W(t)}{(t-s)^{3}},\\qquad\n\tW(t)=\\dfrac{24}{5}t^{5}+24t^{3}+56t+\\dfrac{72}{t}+\\dfrac{48}{t^{3}}+\\dfrac{64}{5t^{5}} .\n\t\\tag{5}\n\t\\]\n\n4.  Volume of the solid of revolution  \n\n\tWhen $\\Sigma(PQ)$ is revolved about $y=-a$, the centroid travels on a circle of radius $\\bar Y+a$.  By Pappus’ centroid theorem,\n\t\\[\n\tV(t)=2\\pi\\,A(t)\\,(\\bar Y+a)\n\t     =\\dfrac{2\\pi a^{3}}{3}\\,\\Phi(t),\n\t\\tag{6}\n\t\\]\n\twhere  \n\t\\[\n\t\\Phi(t)=W(t)+(t-s)^{3}\n\t       =\\dfrac{24}{5}t^{5}+32t^{3}+80t+\\dfrac{96}{t}+\\dfrac{56}{t^{3}}+\\dfrac{64}{5t^{5}} .\n\t\\tag{7}\n\t\\]\n\tBecause the factor $\\dfrac{2\\pi a^{3}}{3}$ is positive, minimising $V(t)$ reduces to minimising $\\Phi(t)$ for $t>0$.\n\n5.  Analysis of $\\Phi$  \n\n\t\\[\n\t\\Phi'(t)=24t^{4}+96t^{2}+80-\\dfrac{96}{t^{2}}-\\dfrac{168}{t^{4}}-\\dfrac{64}{t^{6}},\n\t\\tag{8}\n\t\\]\n\t\\[\n\t\\Phi''(t)=96t^{3}+192t+\\dfrac{192}{t^{3}}+\\dfrac{672}{t^{5}}+\\dfrac{384}{t^{7}}>0\n\t          \\quad\\forall\\,t>0 .\n\t\\tag{9}\n\t\\]\n\tThus $\\Phi$ is strictly convex on $(0,\\infty)$.  Since $\\Phi'(t)$ tends to $-\\infty$ as $t\\to0^{+}$ and to $+\\infty$ as $t\\to\\infty$, it has a unique zero $t_{0}$, which is the sole global minimiser.\n\n6.  Algebraic determination of $t_{0}$  \n\n\tMultiplying (8) by $t^{6}$ and substituting $u=t^{2}$ gives  \n\t\\[\n\t24u^{5}+96u^{4}+80u^{3}-96u^{2}-168u-64=0\n\t\\Longrightarrow\n\t3u^{5}+12u^{4}+10u^{3}-12u^{2}-21u-8=0 .\n\t\\]\n\tLet  \n\t\\[\n\tP(u):=3u^{5}+12u^{4}+10u^{3}-12u^{2}-21u-8.\n\t\\tag{10}\n\t\\]\n\tThe equation $P(u)=0$ has exactly one positive root $u_{0}$; its numerical value is  \n\t\\[\n\tu_{0}\\approx1.207\\,948,\\qquad\n\tt_{0}=\\sqrt{u_{0}}\\approx1.098\\,158.\n\t\\tag{11}\n\t\\]\n\n7.  Closed form of $\\Phi(t_{0})$  \n\n\tUsing (7) and $u=t^{2}$ one has  \n\t\\[\n\t\\Phi(t)=\\dfrac{(24/5)u^{5}+32u^{4}+80u^{3}+96u^{2}+56u+64/5}{u^{5/2}} .\n\t\\]\n\tBecause $P(u)=0$ we may substitute  \n\t\\[\n\tu^{5}= -\\dfrac{12u^{4}+10u^{3}-12u^{2}-21u-8}{3},\n\t\\]\n\twhich after simplification yields the {\\em correct} compact expression  \n\t\\[\n\t\\boxed{\\;\n\t\\Phi(t_{0})=\n\t     \\dfrac{64}{5}\n\t     \\Bigl(\n\t       t_{0}^{3}+5t_{0}\n\t       +\\dfrac{9}{t_{0}}\n\t       +\\dfrac{7}{t_{0}^{3}}\n\t       +\\dfrac{2}{t_{0}^{5}}\n\t     \\Bigr)}\n\t\\tag{12}\n\t\\]\n\t(or, equivalently, $\\displaystyle \\dfrac{64}{5}\\bigl(u_{0}^{3/2}+5u_{0}^{1/2}+9u_{0}^{-1/2}+7u_{0}^{-3/2}+2u_{0}^{-5/2}\\bigr)$).  \n\tNumerically  \n\t\\[\n\t\\Phi(t_{0})\\approx275.63 .\n\t\\]\n\n8.  The optimal chord and the minimal volume  \n\n\tThe optimal normal chord is determined by  \n\t\\[\n\ts_{0}=-t_{0}-\\dfrac{2}{t_{0}},\\qquad\n\tP_{\\min}=P(t_{0})=(2at_{0},\\,at_{0}^{2}).\n\t\\tag{13}\n\t\\]\n\tSubstituting (12) into (6) gives  \n\t\\[\n\tV_{\\min}\n\t   =\\dfrac{2\\pi a^{3}}{3}\\,\\Phi(t_{0})\n\t   =\\dfrac{128\\pi a^{3}}{15}\\!\n\t     \\left(\n\t       t_{0}^{3}+5t_{0}\n\t       +\\dfrac{9}{t_{0}}\n\t       +\\dfrac{7}{t_{0}^{3}}\n\t       +\\dfrac{2}{t_{0}^{5}}\n\t     \\right)\n\t   \\approx1.838\\times10^{2}\\,\\pi a^{3}.\n\t\\tag{14}\n\t\\]\n\n\tBecause the parabola is symmetric about the $y$–axis, the reflected point $(-2at_{0},\\,at_{0}^{2})$ yields the second optimal chord with the same minimal volume.\n\n──────────────────────────────────  \n\nAnswer.  \n\n1.  The minimal solid of revolution is obtained by the normal chord whose lower end–point is  \n\t\\[\n\t\\boxed{\\;\n\t  P_{\\min}=(2at_{0},\\,at_{0}^{2}),\\qquad\n\t  t_{0}>0\\text{ defined by }3t_{0}^{10}+12t_{0}^{8}+10t_{0}^{6}-12t_{0}^{4}-21t_{0}^{2}-8=0\n\t \\;}\n\t\\]\n\t(together with its mirror image about the $y$–axis).\n\n2.  The corresponding minimal volume equals  \n\t\\[\n\t\\boxed{\\;\n\t  V_{\\min}= \\dfrac{128\\pi a^{3}}{15}\\!\n\t            \\left(\n\t              t_{0}^{3}+5t_{0}\n\t              +\\dfrac{9}{t_{0}}\n\t              +\\dfrac{7}{t_{0}^{3}}\n\t              +\\dfrac{2}{t_{0}^{5}}\n\t            \\right)\n\t          \\approx 1.838\\times10^{2}\\,\\pi a^{3}\n\t \\;}\n\t\\]\n\twhere $t_{0}\\approx1.098\\,158$ is the unique positive solution of the quoted decic equation.\n\n──────────────────────────────────",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.438682",
        "was_fixed": false,
        "difficulty_analysis": "The new problem is far harder than both the original and the kernel variant for several independent reasons.\n\n1. Higher-dimensional geometry  \n   – The task involves a three–dimensional solid of revolution instead of a planar segment, forcing the use of spatial geometric tools such as Pappus’s centroid theorem.\n\n2. Added analytic complexity  \n   – Besides the area, the y–coordinate of the centroid of an obliquely-cut parabolic segment is required.  This leads to the evaluation of fourth-degree polynomial integrals and the manipulation of high–degree symmetric expressions.\n\n3. Coupled optimisation  \n   – The volume depends simultaneously on the area and the centroid location, so minimising it is no longer equivalent to minimising a single simple parameter (like the height in the original problem).  A new sixth-degree rational function must be analysed.\n\n4. Non–trivial calculus  \n   – The derivative that must be studied is a tenth–degree polynomial in t; proving uniqueness of the minimiser needs a sign analysis of its derivative rather than a quick inequality.\n\n5. Deeper insight  \n   – Recognising that the centroid lies on the axis of symmetry, exploiting the directrix as the axis of rotation, and keeping the calculation tractable all require a substantially broader set of geometric ideas than the original exercise.\n\nFor these reasons the enhanced kernel variant represents a significant escalation in both technical detail and conceptual depth."
      }
    },
    "original_kernel_variant": {
      "question": "Consider the upward–opening parabola  \n\\[\nx^{2}=4ay , \\qquad a>0 ,\n\\]  \nwhose focus is $F(0,a)$ and whose directrix is the horizontal line $y=-a$.\n\nFor every chord $PQ$ of the parabola the end–point with the smaller $y$–coordinate is denoted by $P$.  \nA chord is called normal if it is perpendicular to the tangent to the parabola at $P$.\n\nLet $\\Sigma(PQ)$ be the finite parabolic segment bounded by the arc $\\widehat{PQ}$ and the chord $PQ$.  \nRotate $\\Sigma(PQ)$ about the directrix $y=-a$ and denote the resulting solid of revolution by $K(P)$.\n\n1.  Among all normal chords $PQ$ determine the position of the point $P$ for which the volume $V(P)$ of $K(P)$ is minimal.  \n2.  Express that minimal volume explicitly in terms of $a$.\n\n(Obviously there will be two symmetric optimal chords, situated on the left and on the right branch of the parabola.  It suffices to find one of them.)  \n\n──────────────────────────────────",
      "solution": "Throughout we work on the right branch of the parabola ($x>0$); by symmetry the reflected results hold for the left branch.\n\n1.  Parametrisation and the normality condition  \n\n\tIntroduce the standard parameter  \n\t\\[\n\t(x,y)=(2au,\\,au^{2}),\\qquad u\\in\\mathbb{R}.\n\t\\tag{0}\n\t\\]  \n\tLet  \n\t\\[\n\tP=P(t)=(2at,\\,at^{2}), \\quad t>0, \\qquad  \n\tQ=Q(s)=(2as,\\,as^{2}), \\quad s<0 .\n\t\\]\n\tThe tangent at $P$ has slope $t$, so the normal through $P$ has slope $-\\dfrac1t$.  \n\tThe slope of the chord $PQ$ equals $\\dfrac12\\,(t+s)$, hence the normality requirement\n\t\\[\n\t\\dfrac12\\,(t+s)=-\\dfrac1t\\quad\\Longrightarrow\\quad\n\ts=-t-\\dfrac{2}{t}.\n\t\\tag{1}\n\t\\]\n\tConsequently every normal chord is uniquely determined by $t>0$.  Frequently used identities are\n\t\\[\n\tt-s =2\\!\\left(t+\\dfrac1t\\right),\\qquad\n\tt+s=-\\dfrac{2}{t},\\qquad\n\tst=-(t^{2}+2).\n\t\\tag{2}\n\t\\]\n\n2.  Area of the generating parabolic segment  \n\n\tThe line $PQ$ is $y=\\dfrac12(t+s)x-ast$.  Direct integration gives\n\t\\[\n\tA(t)\n\t    =\\int_{x_1}^{x_2}\\!\\Bigl[\\bigl(\\tfrac12(t+s)x-ast\\bigr)-\\tfrac{x^{2}}{4a}\\Bigr]\\,{\\rm d}x\n\t    =\\dfrac{a^{2}}{3}\\,(t-s)^{3}>0 .\n\t\\tag{3}\n\t\\]\n\n3.  $y$–coordinate of the centroid of $\\Sigma(PQ)$  \n\n\tDenote the centroid by $G=(\\bar X,\\bar Y)$.  Using vertical slices,\n\t\\[\n\t\\bar Y\\,A(t)=\\dfrac12\\int_{x_1}^{x_2}\\!\n\t               \\Bigl[\\bigl(\\tfrac12(t+s)x-ast\\bigr)^{2}-\\Bigl(\\dfrac{x^{2}}{4a}\\Bigr)^{2}\\Bigr]\n\t             \\,{\\rm d}x .\n\t\\tag{4}\n\t\\]\n\tStraightforward evaluation and insertion of (1)–(2) yields  \n\t\\[\n\t\\bar Y\n\t   =a\\,\\dfrac{W(t)}{(t-s)^{3}},\\qquad\n\tW(t)=\\dfrac{24}{5}t^{5}+24t^{3}+56t+\\dfrac{72}{t}+\\dfrac{48}{t^{3}}+\\dfrac{64}{5t^{5}} .\n\t\\tag{5}\n\t\\]\n\n4.  Volume of the solid of revolution  \n\n\tWhen $\\Sigma(PQ)$ is revolved about $y=-a$, the centroid travels on a circle of radius $\\bar Y+a$.  By Pappus’ centroid theorem,\n\t\\[\n\tV(t)=2\\pi\\,A(t)\\,(\\bar Y+a)\n\t     =\\dfrac{2\\pi a^{3}}{3}\\,\\Phi(t),\n\t\\tag{6}\n\t\\]\n\twhere  \n\t\\[\n\t\\Phi(t)=W(t)+(t-s)^{3}\n\t       =\\dfrac{24}{5}t^{5}+32t^{3}+80t+\\dfrac{96}{t}+\\dfrac{56}{t^{3}}+\\dfrac{64}{5t^{5}} .\n\t\\tag{7}\n\t\\]\n\tBecause the factor $\\dfrac{2\\pi a^{3}}{3}$ is positive, minimising $V(t)$ reduces to minimising $\\Phi(t)$ for $t>0$.\n\n5.  Analysis of $\\Phi$  \n\n\t\\[\n\t\\Phi'(t)=24t^{4}+96t^{2}+80-\\dfrac{96}{t^{2}}-\\dfrac{168}{t^{4}}-\\dfrac{64}{t^{6}},\n\t\\tag{8}\n\t\\]\n\t\\[\n\t\\Phi''(t)=96t^{3}+192t+\\dfrac{192}{t^{3}}+\\dfrac{672}{t^{5}}+\\dfrac{384}{t^{7}}>0\n\t          \\quad\\forall\\,t>0 .\n\t\\tag{9}\n\t\\]\n\tThus $\\Phi$ is strictly convex on $(0,\\infty)$.  Since $\\Phi'(t)$ tends to $-\\infty$ as $t\\to0^{+}$ and to $+\\infty$ as $t\\to\\infty$, it has a unique zero $t_{0}$, which is the sole global minimiser.\n\n6.  Algebraic determination of $t_{0}$  \n\n\tMultiplying (8) by $t^{6}$ and substituting $u=t^{2}$ gives  \n\t\\[\n\t24u^{5}+96u^{4}+80u^{3}-96u^{2}-168u-64=0\n\t\\Longrightarrow\n\t3u^{5}+12u^{4}+10u^{3}-12u^{2}-21u-8=0 .\n\t\\]\n\tLet  \n\t\\[\n\tP(u):=3u^{5}+12u^{4}+10u^{3}-12u^{2}-21u-8.\n\t\\tag{10}\n\t\\]\n\tThe equation $P(u)=0$ has exactly one positive root $u_{0}$; its numerical value is  \n\t\\[\n\tu_{0}\\approx1.207\\,948,\\qquad\n\tt_{0}=\\sqrt{u_{0}}\\approx1.098\\,158.\n\t\\tag{11}\n\t\\]\n\n7.  Closed form of $\\Phi(t_{0})$  \n\n\tUsing (7) and $u=t^{2}$ one has  \n\t\\[\n\t\\Phi(t)=\\dfrac{(24/5)u^{5}+32u^{4}+80u^{3}+96u^{2}+56u+64/5}{u^{5/2}} .\n\t\\]\n\tBecause $P(u)=0$ we may substitute  \n\t\\[\n\tu^{5}= -\\dfrac{12u^{4}+10u^{3}-12u^{2}-21u-8}{3},\n\t\\]\n\twhich after simplification yields the {\\em correct} compact expression  \n\t\\[\n\t\\boxed{\\;\n\t\\Phi(t_{0})=\n\t     \\dfrac{64}{5}\n\t     \\Bigl(\n\t       t_{0}^{3}+5t_{0}\n\t       +\\dfrac{9}{t_{0}}\n\t       +\\dfrac{7}{t_{0}^{3}}\n\t       +\\dfrac{2}{t_{0}^{5}}\n\t     \\Bigr)}\n\t\\tag{12}\n\t\\]\n\t(or, equivalently, $\\displaystyle \\dfrac{64}{5}\\bigl(u_{0}^{3/2}+5u_{0}^{1/2}+9u_{0}^{-1/2}+7u_{0}^{-3/2}+2u_{0}^{-5/2}\\bigr)$).  \n\tNumerically  \n\t\\[\n\t\\Phi(t_{0})\\approx275.63 .\n\t\\]\n\n8.  The optimal chord and the minimal volume  \n\n\tThe optimal normal chord is determined by  \n\t\\[\n\ts_{0}=-t_{0}-\\dfrac{2}{t_{0}},\\qquad\n\tP_{\\min}=P(t_{0})=(2at_{0},\\,at_{0}^{2}).\n\t\\tag{13}\n\t\\]\n\tSubstituting (12) into (6) gives  \n\t\\[\n\tV_{\\min}\n\t   =\\dfrac{2\\pi a^{3}}{3}\\,\\Phi(t_{0})\n\t   =\\dfrac{128\\pi a^{3}}{15}\\!\n\t     \\left(\n\t       t_{0}^{3}+5t_{0}\n\t       +\\dfrac{9}{t_{0}}\n\t       +\\dfrac{7}{t_{0}^{3}}\n\t       +\\dfrac{2}{t_{0}^{5}}\n\t     \\right)\n\t   \\approx1.838\\times10^{2}\\,\\pi a^{3}.\n\t\\tag{14}\n\t\\]\n\n\tBecause the parabola is symmetric about the $y$–axis, the reflected point $(-2at_{0},\\,at_{0}^{2})$ yields the second optimal chord with the same minimal volume.\n\n──────────────────────────────────  \n\nAnswer.  \n\n1.  The minimal solid of revolution is obtained by the normal chord whose lower end–point is  \n\t\\[\n\t\\boxed{\\;\n\t  P_{\\min}=(2at_{0},\\,at_{0}^{2}),\\qquad\n\t  t_{0}>0\\text{ defined by }3t_{0}^{10}+12t_{0}^{8}+10t_{0}^{6}-12t_{0}^{4}-21t_{0}^{2}-8=0\n\t \\;}\n\t\\]\n\t(together with its mirror image about the $y$–axis).\n\n2.  The corresponding minimal volume equals  \n\t\\[\n\t\\boxed{\\;\n\t  V_{\\min}= \\dfrac{128\\pi a^{3}}{15}\\!\n\t            \\left(\n\t              t_{0}^{3}+5t_{0}\n\t              +\\dfrac{9}{t_{0}}\n\t              +\\dfrac{7}{t_{0}^{3}}\n\t              +\\dfrac{2}{t_{0}^{5}}\n\t            \\right)\n\t          \\approx 1.838\\times10^{2}\\,\\pi a^{3}\n\t \\;}\n\t\\]\n\twhere $t_{0}\\approx1.098\\,158$ is the unique positive solution of the quoted decic equation.\n\n──────────────────────────────────",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.378868",
        "was_fixed": false,
        "difficulty_analysis": "The new problem is far harder than both the original and the kernel variant for several independent reasons.\n\n1. Higher-dimensional geometry  \n   – The task involves a three–dimensional solid of revolution instead of a planar segment, forcing the use of spatial geometric tools such as Pappus’s centroid theorem.\n\n2. Added analytic complexity  \n   – Besides the area, the y–coordinate of the centroid of an obliquely-cut parabolic segment is required.  This leads to the evaluation of fourth-degree polynomial integrals and the manipulation of high–degree symmetric expressions.\n\n3. Coupled optimisation  \n   – The volume depends simultaneously on the area and the centroid location, so minimising it is no longer equivalent to minimising a single simple parameter (like the height in the original problem).  A new sixth-degree rational function must be analysed.\n\n4. Non–trivial calculus  \n   – The derivative that must be studied is a tenth–degree polynomial in t; proving uniqueness of the minimiser needs a sign analysis of its derivative rather than a quick inequality.\n\n5. Deeper insight  \n   – Recognising that the centroid lies on the axis of symmetry, exploiting the directrix as the axis of rotation, and keeping the calculation tractable all require a substantially broader set of geometric ideas than the original exercise.\n\nFor these reasons the enhanced kernel variant represents a significant escalation in both technical detail and conceptual depth."
      }
    }
  },
  "checked": true,
  "problem_type": "calculation",
  "iteratively_fixed": true
}