{
  "index": "1954-A-3",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "3. Prove that if the family of integral curves of the differential equation\n\\[\n\\frac{d y}{d x}+p(x) y=q(x) \\quad p(x) \\cdot q(x) \\neq 0\n\\]\nis cut by the line \\( x=k \\), the tangents at the points of intersection are concurrent.",
  "solution": "Solution. The equation of the line tangent to the smooth curve \\( y=f(x) \\) at the point \\( (k, m) \\) is\n\\[\ny-m=f^{\\prime}(k)(x-k)\n\\]\n\nIf \\( f \\) is a solution of the given differential equation this becomes\n\\[\ny-m=[q(k)-m p(k)](x-k)\n\\]\n\nFor any value of \\( m \\), this line passes through the point\n\\[\n\\left(k+\\frac{1}{p(k)}, \\frac{q(k)}{p(k)}\\right)\n\\]",
  "vars": [
    "x",
    "y",
    "f",
    "m"
  ],
  "params": [
    "p",
    "q",
    "k"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "abscissa",
        "y": "ordinate",
        "f": "curvefn",
        "m": "valueat",
        "p": "coefone",
        "q": "coeftwo",
        "k": "cutline"
      },
      "question": "3. Prove that if the family of integral curves of the differential equation\n\\[\n\\frac{d\\, ordinate}{d\\, abscissa}+coefone(abscissa)\\, ordinate = coeftwo(abscissa) \\quad coefone(abscissa) \\cdot coeftwo(abscissa) \\neq 0\n\\]\nis cut by the line \\( abscissa = cutline \\), the tangents at the points of intersection are concurrent.",
      "solution": "Solution. The equation of the line tangent to the smooth curve \\( ordinate = curvefn(abscissa) \\) at the point \\( (cutline, valueat) \\) is\n\\[\nordinate - valueat = curvefn^{\\prime}(cutline)(abscissa - cutline)\n\\]\n\nIf \\( curvefn \\) is a solution of the given differential equation this becomes\n\\[\nordinate - valueat = [ coeftwo(cutline) - valueat\\, coefone(cutline) ] ( abscissa - cutline )\n\\]\n\nFor any value of \\( valueat \\), this line passes through the point\n\\[\n\\left( cutline + \\frac{1}{ coefone(cutline) }, \\frac{ coeftwo(cutline) }{ coefone(cutline) } \\right)\n\\]"
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "paperclip",
        "y": "toothbrush",
        "f": "caterpillar",
        "m": "chandelier",
        "p": "lumberjack",
        "q": "marzipans",
        "k": "horseshoe"
      },
      "question": "3. Prove that if the family of integral curves of the differential equation\n\\[\n\\frac{d toothbrush}{d paperclip}+lumberjack(paperclip) toothbrush=marzipans(paperclip) \\quad lumberjack(paperclip) \\cdot marzipans(paperclip) \\neq 0\n\\]\nis cut by the line \\( paperclip=horseshoe \\), the tangents at the points of intersection are concurrent.",
      "solution": "Solution. The equation of the line tangent to the smooth curve \\( toothbrush=caterpillar(paperclip) \\) at the point \\( (horseshoe, chandelier) \\) is\n\\[\ntoothbrush-chandelier=caterpillar^{\\prime}(horseshoe)(paperclip-horseshoe)\n\\]\n\nIf \\( caterpillar \\) is a solution of the given differential equation this becomes\n\\[\ntoothbrush-chandelier=[marzipans(horseshoe)-chandelier lumberjack(horseshoe)](paperclip-horseshoe)\n\\]\n\nFor any value of \\( chandelier \\), this line passes through the point\n\\[\n\\left(horseshoe+\\frac{1}{lumberjack(horseshoe)}, \\frac{marzipans(horseshoe)}{lumberjack(horseshoe)}\\right)\n\\]"
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "verticalaxis",
        "y": "horizontalaxis",
        "f": "constantvalue",
        "m": "dynamicdip",
        "p": "steadystate",
        "q": "zeroquantity",
        "k": "variablepoint"
      },
      "question": "3. Prove that if the family of integral curves of the differential equation\n\\[\n\\frac{d horizontalaxis}{d verticalaxis}+steadystate(verticalaxis) horizontalaxis=zeroquantity(verticalaxis) \\quad steadystate(verticalaxis) \\cdot zeroquantity(verticalaxis) \\neq 0\n\\]\nis cut by the line \\( verticalaxis=variablepoint \\), the tangents at the points of intersection are concurrent.",
      "solution": "Solution. The equation of the line tangent to the smooth curve \\( horizontalaxis=constantvalue(verticalaxis) \\) at the point \\( (variablepoint, dynamicdip) \\) is\n\\[\nhorizontalaxis-dynamicdip=constantvalue^{\\prime}(variablepoint)(verticalaxis-variablepoint)\n\\]\n\nIf constantvalue is a solution of the given differential equation this becomes\n\\[\nhorizontalaxis-dynamicdip=[zeroquantity(variablepoint)-dynamicdip\\; steadystate(variablepoint)](verticalaxis-variablepoint)\n\\]\n\nFor any value of dynamicdip, this line passes through the point\n\\[\n\\left(variablepoint+\\frac{1}{steadystate(variablepoint)}, \\frac{zeroquantity(variablepoint)}{steadystate(variablepoint)}\\right)\n\\]"
    },
    "garbled_string": {
      "map": {
        "x": "tqabvzye",
        "y": "wfcznuro",
        "f": "glsproqe",
        "m": "xhvyreot",
        "p": "qrsdplak",
        "q": "zmytcoha",
        "k": "jvwdurto"
      },
      "question": "3. Prove that if the family of integral curves of the differential equation\n\\[\n\\frac{d wfcznuro}{d tqabvzye}+qrsdplak(tqabvzye) wfcznuro=zmytcoha(tqabvzye) \\quad qrsdplak(tqabvzye) \\cdot zmytcoha(tqabvzye) \\neq 0\n\\]\nis cut by the line \\( tqabvzye=jvwdurto \\), the tangents at the points of intersection are concurrent.",
      "solution": "Solution. The equation of the line tangent to the smooth curve \\( wfcznuro=glsproqe(tqabvzye) \\) at the point \\( (jvwdurto, xhvyreot) \\) is\n\\[\nwfcznuro-xhvyreot=glsproqe^{\\prime}(jvwdurto)(tqabvzye-jvwdurto)\n\\]\n\nIf \\( glsproqe \\) is a solution of the given differential equation this becomes\n\\[\nwfcznuro-xhvyreot=[zmytcoha(jvwdurto)-xhvyreot qrsdplak(jvwdurto)](tqabvzye-jvwdurto)\n\\]\n\nFor any value of \\( xhvyreot \\), this line passes through the point\n\\[\n\\left(jvwdurto+\\frac{1}{qrsdplak(jvwdurto)}, \\frac{zmytcoha(jvwdurto)}{qrsdplak(jvwdurto)}\\right)\n\\]"
    },
    "kernel_variant": {
      "question": "Let $I\\subset\\mathbb R$ be an open interval containing the fixed point $a$.  \nFor an integer $n\\ge 1$ let  \n\n  $A:I\\longrightarrow M_{n}(\\mathbb R),\\qquad b:I\\longrightarrow\\mathbb R^{n}$  \n\nbe $C^{1}$–maps, and consider the inhomogeneous linear system  \n\n(1)  $Y'(x)=A(x)\\,Y(x)+b(x)\\qquad (x\\in I),\\qquad Y:I\\to\\mathbb R^{n}\\;.$  \n\nEvery solution $Y$ defines a space–curve  \n  $\\Gamma_{Y}:x\\longmapsto\\bigl(x,Y(x)\\bigr)\\subset\\mathbb R^{\\,n+1}$.  \nDenote by $\\ell_{Y}$ the tangent line to $\\Gamma_{Y}$ at the point  \n$P_{Y}:=(a,Y(a))$.\n\n(a)  Show that the family $\\{\\ell_{Y}\\}_{Y\\text{ solves (1)}}$ of lines is a pencil\n     (i.e. all the lines pass through one common point) iff the constant matrix\n     $A(a)$ is a non-zero scalar multiple of the identity:\n             $\\displaystyle A(a)=p\\,I_{n}\\;(p\\ne0).$\n\n(b)  If the equivalent condition in (a) holds, determine the common intersection\n     point $P^{\\!*}$ of all the lines $\\ell_{Y}$ explicitly in terms of $p$ and\n     $b(a)$.\n\n(c)  Give a concrete $2\\times2$ example in which $A(a)$ is not a scalar matrix\n     and verify directly (without appealing to (a)) that the corresponding\n     tangent lines are **not** concurrent.",
      "solution": "Throughout write  \n  $A_{0}:=A(a)\\in M_{n}(\\mathbb R),\\qquad b_{0}:=b(a)\\in\\mathbb R^{n}.$  \n\nStep 1.  An equation for $\\ell_{Y}$.  \nFor an arbitrary solution $Y$ put $Y_{0}:=Y(a)$.  From (1) we have  \n  $Y'(a)=A_{0}Y_{0}+b_{0}=:V_{0}.$  \nHence the tangent line at $P_{Y}=(a,Y_{0})$ is  \n\n(2)  $\\ell_{Y}(t):=(a,Y_{0})+t\\,(1,V_{0})\\qquad(t\\in\\mathbb R).$\n\nStep 2.  A necessary algebraic condition for concurrence.  \nAssume there exists a point  \n  $P^{\\!*}=(x^{\\!*},y^{\\!*})\\in\\mathbb R^{\\,n+1}$  \nsuch that $P^{\\!*}\\in\\ell_{Y}$ for **every** solution $Y$.  \nFor a particular solution the parameter value $t$ needed in (2) is forced by\nthe $x$–coordinate:\n\n(3)  $t=x^{\\!*}-a=:t^{\\!*}\\quad\\text{(same $t^{\\!*}$ for all $Y$).}$  \n\nUsing (2)–(3) the $y$–coordinates give  \n\n(4)  $y^{\\!*}=Y_{0}+t^{\\!*}\\,(A_{0}Y_{0}+b_{0})\n         =\\bigl(I_{n}+t^{\\!*}A_{0}\\bigr)Y_{0}+t^{\\!*}b_{0}\\quad\n           \\text{for \\emph{every}}\\,Y_{0}\\in\\mathbb R^{n}.$\n\nIndependence of $Y_{0}$ forces  \n\n(5)  $I_{n}+t^{\\!*}A_{0}=0\\quad\\Longrightarrow\\quad\n         A_{0}=-\\frac1{t^{\\!*}}\\,I_{n}=:p\\,I_{n},\\;p\\ne0.$\n\nThus $A(a)$ must be a non-zero scalar matrix.  This proves the “only if’’ part\nof (a).\n\nStep 3.  Sufficiency.  \nConversely suppose $A_{0}=p\\,I_{n}$ with $p\\ne0$.  Choose  \n\n(6)  $t^{\\!*}:=-\\dfrac1{p},\\qquad \n         P^{\\!*}:=\\bigl(a+t^{\\!*},\\,y^{\\!*}\\bigr),\\;\n         y^{\\!*}:=-\\dfrac{b_{0}}{p}.$\n\nFor an arbitrary solution $Y$ formula (2) with $t=t^{\\!*}$ yields  \n\n$(a+t^{\\!*},\\,Y_{0}+t^{\\!*}(pY_{0}+b_{0}))\n=(a+t^{\\!*},\\,-\\tfrac{b_{0}}{p})=P^{\\!*},$\n\nso $P^{\\!*}\\in\\ell_{Y}$.  Hence all lines are concurrent at $P^{\\!*}$, proving\nthe “if’’ part and completing (a)–(b).\n\nStep 4.  A non-concurrent example (part (c)).  \nLet $n=2$, choose  \n\n$A(x)=\\begin{pmatrix}1&1\\\\0&2\\end{pmatrix}$ (so $A(a)=A_{0}$ is \\emph{not}\na scalar matrix) and $b(x)\\equiv(0,0)^{T}$.  \nFor initial data $Y_{0}=(y_{1},y_{2})^{T}$ one solves (1) near $x=a$:\n\n$Y'(a)=A_{0}Y_{0}=(y_{1}+y_{2},\\,2y_{2})^{T}.$  \n\nThe tangent line (2) becomes  \n\n$(x,y_{1},y_{2})=(a,y_{1},y_{2})+t\\,(1,y_{1}+y_{2},2y_{2}).$\n\nTaking two particular starting points, say $Y_{0}^{(1)}=(1,0)^{T}$ and\n$Y_{0}^{(2)}=(0,1)^{T}$, one obtains the distinct tangent lines  \n\n$\\ell_{1}:\\;(x,y_{1},y_{2})=(a,1,0)+t\\,(1,1,0),$  \n\n$\\ell_{2}:\\;(x,y_{1},y_{2})=(a,0,1)+t\\,(1,1,2).$  \n\nSolving the linear system for a putative intersection point shows that\nno common solution $(x,y_{1},y_{2})$ exists, so the lines are not concurrent,\nas predicted by part (a).",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.461314",
        "was_fixed": false,
        "difficulty_analysis": "• The original problem involves a single first-order scalar ODE and planar\n  geometry; a single differentiation and a one–line substitution give the\n  answer.  \n• The enhanced variant replaces the scalar ODE by a \\emph{vector} inhomogeneous\n  system, embeds the curves in the higher-dimensional space\n  $\\mathbb R^{\\,n+1}$, and asks for a complete characterisation\n  (necessary \\emph{and} sufficient conditions) of when concurrency occurs.  \n  This forces the solver to  \n  – write the tangent lines in vector form,  \n  – analyse the resulting condition (4) for \\emph{all} initial vectors\n    $Y_{0}$,  \n  – recognise and solve the matrix equation\n    $I_{n}+tA_{0}=0$,  \n  – connect the algebraic outcome to geometric concurrency, and  \n  – produce a counter-example when the condition fails.  \n• These steps require linear algebra over matrices (invertibility,\n  identification of scalar matrices), elementary but non-trivial projective\n  geometry in $\\mathbb R^{\\,n+1}$, and explicit construction of solutions to\n  verify the negative case.  \n• Even for $n=2$ the general condition is not obvious; for arbitrary $n$ the\n  solver must argue abstractly rather than by direct coordinates.\n  Hence the problem is substantially more sophisticated than the original and\n  cannot be dispatched by simple pattern matching."
      }
    },
    "original_kernel_variant": {
      "question": "Let $I\\subset\\mathbb R$ be an open interval containing the fixed point $a$.  \nFor an integer $n\\ge 1$ let  \n\n  $A:I\\longrightarrow M_{n}(\\mathbb R),\\qquad b:I\\longrightarrow\\mathbb R^{n}$  \n\nbe $C^{1}$–maps, and consider the inhomogeneous linear system  \n\n(1)  $Y'(x)=A(x)\\,Y(x)+b(x)\\qquad (x\\in I),\\qquad Y:I\\to\\mathbb R^{n}\\;.$  \n\nEvery solution $Y$ defines a space–curve  \n  $\\Gamma_{Y}:x\\longmapsto\\bigl(x,Y(x)\\bigr)\\subset\\mathbb R^{\\,n+1}$.  \nDenote by $\\ell_{Y}$ the tangent line to $\\Gamma_{Y}$ at the point  \n$P_{Y}:=(a,Y(a))$.\n\n(a)  Show that the family $\\{\\ell_{Y}\\}_{Y\\text{ solves (1)}}$ of lines is a pencil\n     (i.e. all the lines pass through one common point) iff the constant matrix\n     $A(a)$ is a non-zero scalar multiple of the identity:\n             $\\displaystyle A(a)=p\\,I_{n}\\;(p\\ne0).$\n\n(b)  If the equivalent condition in (a) holds, determine the common intersection\n     point $P^{\\!*}$ of all the lines $\\ell_{Y}$ explicitly in terms of $p$ and\n     $b(a)$.\n\n(c)  Give a concrete $2\\times2$ example in which $A(a)$ is not a scalar matrix\n     and verify directly (without appealing to (a)) that the corresponding\n     tangent lines are **not** concurrent.",
      "solution": "Throughout write  \n  $A_{0}:=A(a)\\in M_{n}(\\mathbb R),\\qquad b_{0}:=b(a)\\in\\mathbb R^{n}.$  \n\nStep 1.  An equation for $\\ell_{Y}$.  \nFor an arbitrary solution $Y$ put $Y_{0}:=Y(a)$.  From (1) we have  \n  $Y'(a)=A_{0}Y_{0}+b_{0}=:V_{0}.$  \nHence the tangent line at $P_{Y}=(a,Y_{0})$ is  \n\n(2)  $\\ell_{Y}(t):=(a,Y_{0})+t\\,(1,V_{0})\\qquad(t\\in\\mathbb R).$\n\nStep 2.  A necessary algebraic condition for concurrence.  \nAssume there exists a point  \n  $P^{\\!*}=(x^{\\!*},y^{\\!*})\\in\\mathbb R^{\\,n+1}$  \nsuch that $P^{\\!*}\\in\\ell_{Y}$ for **every** solution $Y$.  \nFor a particular solution the parameter value $t$ needed in (2) is forced by\nthe $x$–coordinate:\n\n(3)  $t=x^{\\!*}-a=:t^{\\!*}\\quad\\text{(same $t^{\\!*}$ for all $Y$).}$  \n\nUsing (2)–(3) the $y$–coordinates give  \n\n(4)  $y^{\\!*}=Y_{0}+t^{\\!*}\\,(A_{0}Y_{0}+b_{0})\n         =\\bigl(I_{n}+t^{\\!*}A_{0}\\bigr)Y_{0}+t^{\\!*}b_{0}\\quad\n           \\text{for \\emph{every}}\\,Y_{0}\\in\\mathbb R^{n}.$\n\nIndependence of $Y_{0}$ forces  \n\n(5)  $I_{n}+t^{\\!*}A_{0}=0\\quad\\Longrightarrow\\quad\n         A_{0}=-\\frac1{t^{\\!*}}\\,I_{n}=:p\\,I_{n},\\;p\\ne0.$\n\nThus $A(a)$ must be a non-zero scalar matrix.  This proves the “only if’’ part\nof (a).\n\nStep 3.  Sufficiency.  \nConversely suppose $A_{0}=p\\,I_{n}$ with $p\\ne0$.  Choose  \n\n(6)  $t^{\\!*}:=-\\dfrac1{p},\\qquad \n         P^{\\!*}:=\\bigl(a+t^{\\!*},\\,y^{\\!*}\\bigr),\\;\n         y^{\\!*}:=-\\dfrac{b_{0}}{p}.$\n\nFor an arbitrary solution $Y$ formula (2) with $t=t^{\\!*}$ yields  \n\n$(a+t^{\\!*},\\,Y_{0}+t^{\\!*}(pY_{0}+b_{0}))\n=(a+t^{\\!*},\\,-\\tfrac{b_{0}}{p})=P^{\\!*},$\n\nso $P^{\\!*}\\in\\ell_{Y}$.  Hence all lines are concurrent at $P^{\\!*}$, proving\nthe “if’’ part and completing (a)–(b).\n\nStep 4.  A non-concurrent example (part (c)).  \nLet $n=2$, choose  \n\n$A(x)=\\begin{pmatrix}1&1\\\\0&2\\end{pmatrix}$ (so $A(a)=A_{0}$ is \\emph{not}\na scalar matrix) and $b(x)\\equiv(0,0)^{T}$.  \nFor initial data $Y_{0}=(y_{1},y_{2})^{T}$ one solves (1) near $x=a$:\n\n$Y'(a)=A_{0}Y_{0}=(y_{1}+y_{2},\\,2y_{2})^{T}.$  \n\nThe tangent line (2) becomes  \n\n$(x,y_{1},y_{2})=(a,y_{1},y_{2})+t\\,(1,y_{1}+y_{2},2y_{2}).$\n\nTaking two particular starting points, say $Y_{0}^{(1)}=(1,0)^{T}$ and\n$Y_{0}^{(2)}=(0,1)^{T}$, one obtains the distinct tangent lines  \n\n$\\ell_{1}:\\;(x,y_{1},y_{2})=(a,1,0)+t\\,(1,1,0),$  \n\n$\\ell_{2}:\\;(x,y_{1},y_{2})=(a,0,1)+t\\,(1,1,2).$  \n\nSolving the linear system for a putative intersection point shows that\nno common solution $(x,y_{1},y_{2})$ exists, so the lines are not concurrent,\nas predicted by part (a).",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.392128",
        "was_fixed": false,
        "difficulty_analysis": "• The original problem involves a single first-order scalar ODE and planar\n  geometry; a single differentiation and a one–line substitution give the\n  answer.  \n• The enhanced variant replaces the scalar ODE by a \\emph{vector} inhomogeneous\n  system, embeds the curves in the higher-dimensional space\n  $\\mathbb R^{\\,n+1}$, and asks for a complete characterisation\n  (necessary \\emph{and} sufficient conditions) of when concurrency occurs.  \n  This forces the solver to  \n  – write the tangent lines in vector form,  \n  – analyse the resulting condition (4) for \\emph{all} initial vectors\n    $Y_{0}$,  \n  – recognise and solve the matrix equation\n    $I_{n}+tA_{0}=0$,  \n  – connect the algebraic outcome to geometric concurrency, and  \n  – produce a counter-example when the condition fails.  \n• These steps require linear algebra over matrices (invertibility,\n  identification of scalar matrices), elementary but non-trivial projective\n  geometry in $\\mathbb R^{\\,n+1}$, and explicit construction of solutions to\n  verify the negative case.  \n• Even for $n=2$ the general condition is not obvious; for arbitrary $n$ the\n  solver must argue abstractly rather than by direct coordinates.\n  Hence the problem is substantially more sophisticated than the original and\n  cannot be dispatched by simple pattern matching."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}