{
  "index": "1977-A-1",
  "type": "ALG",
  "tag": [
    "ALG"
  ],
  "difficulty": "",
  "question": "Problem A-1\nConsider all lines which meet the graph of\n\\[\ny=2 x^{4}+7 x^{3}+3 x-5\n\\]\nin four distinct points, say \\( \\left(x_{i}, y_{i}\\right), i=1,2,3,4 \\). Show that\n\\[\n\\frac{x_{1}+x_{2}+x_{3}+x_{4}}{4}\n\\]\nis independent of the line and find its value.",
  "solution": "A-1.\nA line meeting the graph in four points has an equation \\( y=m x+b \\). Then the \\( x_{i} \\) are the roots of\n\\[\n2 x^{4}+7 x^{3}+(3-m) x-(5+b)=0\n\\]\ntheir sum is \\( -7 / 2 \\), and their arithmetic mean \\( \\left(\\Sigma x_{i}\\right) / 4 \\) is \\( -7 / 8 \\), which is independent of the line.",
  "vars": [
    "x",
    "x_i",
    "x_1",
    "x_2",
    "x_3",
    "x_4",
    "y",
    "y_i"
  ],
  "params": [
    "m",
    "b"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "varxcoord",
        "x_i": "varxindex",
        "x_1": "varxone",
        "x_2": "varxtwo",
        "x_3": "varxthree",
        "x_4": "varxfour",
        "y": "varycoord",
        "y_i": "varyindex",
        "m": "parammcoef",
        "b": "paramshift"
      },
      "question": "Problem A-1\nConsider all lines which meet the graph of\n\\[\nvarycoord=2 varxcoord^{4}+7 varxcoord^{3}+3 varxcoord-5\n\\]\nin four distinct points, say \\( \\left(varxindex, varyindex\\right), i=1,2,3,4 \\). Show that\n\\[\n\\frac{varxone+varxtwo+varxthree+varxfour}{4}\n\\]\nis independent of the line and find its value.",
      "solution": "A-1.\nA line meeting the graph in four points has an equation \\( varycoord=parammcoef varxcoord+paramshift \\). Then the \\( varxindex \\) are the roots of\n\\[\n2 varxcoord^{4}+7 varxcoord^{3}+(3-parammcoef) varxcoord-(5+paramshift)=0\n\\]\ntheir sum is \\( -7 / 2 \\), and their arithmetic mean \\( \\left(\\Sigma varxindex\\right) / 4 \\) is \\( -7 / 8 \\), which is independent of the line."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "riverbank",
        "x_i": "riverbankindex",
        "x_1": "riverbankalpha",
        "x_2": "riverbankbeta",
        "x_3": "riverbankgamma",
        "x_4": "riverbankdelta",
        "y": "hillside",
        "y_i": "hillsideindex",
        "m": "sailfish",
        "b": "turnpike"
      },
      "question": "Problem A-1\nConsider all lines which meet the graph of\n\\[\nhillside=2 riverbank^{4}+7 riverbank^{3}+3 riverbank-5\n\\]\nin four distinct points, say \\( \\left(riverbankindex, hillsideindex\\right), i=1,2,3,4 \\). Show that\n\\[\n\\frac{riverbankalpha+riverbankbeta+riverbankgamma+riverbankdelta}{4}\n\\]\nis independent of the line and find its value.",
      "solution": "A-1.\nA line meeting the graph in four points has an equation \\( hillside=sailfish riverbank+turnpike \\). Then the \\( riverbankindex \\) are the roots of\n\\[\n2 riverbank^{4}+7 riverbank^{3}+(3-sailfish) riverbank-(5+turnpike)=0\n\\]\ntheir sum is \\( -7 / 2 \\), and their arithmetic mean \\( (\\Sigma riverbankindex) / 4 \\) is \\( -7 / 8 \\), which is independent of the line."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "verticalaxis",
        "x_i": "verticalsample",
        "x_1": "verticalfirst",
        "x_2": "verticalsecond",
        "x_3": "verticalthird",
        "x_4": "verticalfourth",
        "y": "horizontalaxis",
        "y_i": "horizontalsample",
        "m": "flatscalar",
        "b": "divergence"
      },
      "question": "Problem A-1\nConsider all lines which meet the graph of\n\\[\nhorizontalaxis=2 verticalaxis^{4}+7 verticalaxis^{3}+3 verticalaxis-5\n\\]\nin four distinct points, say \\( \\left(verticalsample, horizontalsample\\right), i=1,2,3,4 \\). Show that\n\\[\n\\frac{verticalfirst+verticalsecond+verticalthird+verticalfourth}{4}\n\\]\nis independent of the line and find its value.",
      "solution": "A-1.\nA line meeting the graph in four points has an equation \\( horizontalaxis=flatscalar verticalaxis+divergence \\). Then the \\( verticalsample \\) are the roots of\n\\[\n2 verticalaxis^{4}+7 verticalaxis^{3}+(3-flatscalar) verticalaxis-(5+divergence)=0\n\\]\ntheir sum is \\( -7 / 2 \\), and their arithmetic mean \\( \\left(\\Sigma verticalsample\\right) / 4 \\) is \\( -7 / 8 \\), which is independent of the line."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "x_i": "hjgrksla",
        "x_1": "pqlkmnrz",
        "x_2": "zxcfghjk",
        "x_3": "mnbvrety",
        "x_4": "lkjhgfds",
        "y": "asdkfjgh",
        "y_i": "qweruiop",
        "m": "cvbnmert",
        "b": "ghjklasd"
      },
      "question": "Problem A-1\nConsider all lines which meet the graph of\n\\[\nasdkfjgh=2 qzxwvtnp^{4}+7 qzxwvtnp^{3}+3 qzxwvtnp-5\n\\]\nin four distinct points, say \\( \\left(hjgrksla, qweruiop\\right), i=1,2,3,4 \\). Show that\n\\[\n\\frac{pqlkmnrz+zxcfghjk+mnbvrety+lkjhgfds}{4}\n\\]\nis independent of the line and find its value.",
      "solution": "A-1.\nA line meeting the graph in four points has an equation \\( asdkfjgh=cvbnmert qzxwvtnp+ghjklasd \\). Then the \\( hjgrksla \\) are the roots of\n\\[\n2 qzxwvtnp^{4}+7 qzxwvtnp^{3}+(3-cvbnmert) qzxwvtnp-(5+ghjklasd)=0\n\\]\ntheir sum is \\( -7 / 2 \\), and their arithmetic mean \\( \\left(\\Sigma hjgrksla\\right) / 4 \\) is \\( -7 / 8 \\), which is independent of the line."
    },
    "kernel_variant": {
      "question": "Let ℓ be a line that meets the graph of\n\\[\n  y = 5x^{4}-9x^{3}+4x^{2}-8x+6\n\\]\nin four distinct real points \\((x_{1},y_{1}),\\ldots,(x_{4},y_{4})\\).  Prove that\n\\[\n  \\frac{x_{1}+x_{2}+x_{3}+x_{4}}{4}\n\\]\nis the same for every such line ℓ, and determine its value.",
      "solution": "Parameterise an arbitrary line ℓ by\n\n   y = mx + b,   m,b∈ℝ.\n\nIntersection abscissas x₁,…,x₄ are the roots of\n\n   5x⁴ − 9x³ + 4x² − 8x + 6 − (mx + b) = 0,\n\ni.e.\n\n   5x⁴ − 9x³ + 4x² + (−8−m)x + (6−b) = 0.  (1)\n\n1. The leading coefficient and the coefficient of x³ in (1) are 5 and −9, neither depending on m or b.\n2. By Vieta’s formula for a quartic ax⁴ + cx³ + … = 0, the sum of the roots is −c/a.  Here that gives\n\n   x₁+x₂+x₃+x₄ = −(−9)/5 = 9/5,\n\nindependent of m,b.\n3. Therefore the required arithmetic mean is\n\n   (x₁+x₂+x₃+x₄)/4 = (1/4)·(9/5) = 9/20.\n\nBecause only the fixed coefficients 5 and −9 enter, this holds for every line meeting the quartic in four distinct real points, completing the proof.\n\n(*One need not normalize to a monic polynomial; Vieta’s formula in the non-monic case yields the same ratio.)",
      "_meta": {
        "core_steps": [
          "Parameterize any intersecting line by y = m x + b.",
          "Set the line equal to the quartic; the x-coordinates satisfy 2x⁴ + 7x³ + 3x − 5 − (mx + b) = 0.",
          "Use Vieta: Σx_i = −(coeff. of x³)/(coeff. of x⁴), a value independent of m and b.",
          "Compute the arithmetic mean as (Σx_i)/4.",
          "Conclude that this mean is constant for all lines."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Leading coefficient of the x⁴ term in the polynomial",
            "original": 2
          },
          "slot2": {
            "description": "Coefficient of the x³ term (the only one affecting Σx_i )",
            "original": 7
          },
          "slot3": {
            "description": "Coefficient of the x² term (currently zero / not present)",
            "original": 0
          },
          "slot4": {
            "description": "Coefficient of the x¹ term",
            "original": 3
          },
          "slot5": {
            "description": "Constant term in the polynomial",
            "original": -5
          },
          "slot6": {
            "description": "Stipulation that the four intersection points be distinct",
            "original": "distinct"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}