{
  "index": "1979-B-1",
  "type": "ANA",
  "tag": [
    "ANA"
  ],
  "difficulty": "",
  "question": "Problem B-1\nProve or disprove: there is at least one straight line normal to the graph of \\( y=\\cosh x \\) at a point \\( (a, \\cosh a) \\) and also normal to the graph of \\( y=\\sinh x \\) at a point \\( (c, \\sinh c) \\).\n[At a point on a graph, the normal line is the perpendicular to the tangent at that point. Also, \\( \\cosh x=\\left(e^{x}+\\right. \\) \\( \\left.e^{-x}\\right) / 2 \\) and \\( \\left.\\sinh x=\\left(e^{x}-e^{-x}\\right) / 2.\\right] \\)",
  "solution": "B-1.\nWe assume that there is such a common normal and obtain a contradiction. This assumption implies\n\\[\n-\\frac{a-c}{\\cosh a-\\sinh c}=\\cosh c=\\sinh a .\n\\]\n\nSince \\( \\cosh x>0 \\) for all real \\( x \\) and \\( \\sinh x>0 \\) only for \\( x>0 \\), (I) implies \\( a>0 \\). Using the fact that \\( \\sinh x<\\cosh x \\) for all \\( x \\) and (I), one obtains\n\\[\n\\sinh c<\\cosh c=\\sinh a<\\cosh a .\n\\]\n\nThis, \\( a>0 \\), and the fact that \\( \\cosh x \\) increases for \\( x>0 \\) imply that \\( c<a \\). Thus the leftmost expression in (I) is negative and cannot equal cosh \\( c \\). This contradiction shows that no common normal exists.",
  "vars": [
    "x",
    "y",
    "a",
    "c"
  ],
  "params": [],
  "sci_consts": [
    "e"
  ],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "horizvar",
        "y": "vertvar",
        "a": "pointone",
        "c": "pointtwo"
      },
      "question": "Problem B-1\nProve or disprove: there is at least one straight line normal to the graph of \\( vertvar=\\cosh horizvar \\) at a point \\( (pointone, \\cosh pointone) \\) and also normal to the graph of \\( vertvar=\\sinh horizvar \\) at a point \\( (pointtwo, \\sinh pointtwo) \\).\n[At a point on a graph, the normal line is the perpendicular to the tangent at that point. Also, \\( \\cosh horizvar=\\left(e^{horizvar}+\\right. \\) \\( \\left.e^{-horizvar}\\right) / 2 \\) and \\( \\left.\\sinh horizvar=\\left(e^{horizvar}-e^{-horizvar}\\right) / 2.\\right] \\)",
      "solution": "B-1.\nWe assume that there is such a common normal and obtain a contradiction. This assumption implies\n\\[\n-\\frac{pointone-pointtwo}{\\cosh pointone-\\sinh pointtwo}=\\cosh pointtwo=\\sinh pointone .\n\\]\n\nSince \\( \\cosh horizvar>0 \\) for all real \\( horizvar \\) and \\( \\sinh horizvar>0 \\) only for \\( horizvar>0 \\), (I) implies \\( pointone>0 \\). Using the fact that \\( \\sinh horizvar<\\cosh horizvar \\) for all \\( horizvar \\) and (I), one obtains\n\\[\n\\sinh pointtwo<\\cosh pointtwo=\\sinh pointone<\\cosh pointone .\n\\]\n\nThis, \\( pointone>0 \\), and the fact that \\( \\cosh horizvar \\) increases for \\( horizvar>0 \\) imply that \\( pointtwo<pointone \\). Thus the leftmost expression in (I) is negative and cannot equal \\cosh pointtwo. This contradiction shows that no common normal exists."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "blueberries",
        "y": "avalanche",
        "a": "drumstick",
        "c": "sailmaker"
      },
      "question": "Problem B-1\nProve or disprove: there is at least one straight line normal to the graph of \\( avalanche=\\cosh blueberries \\) at a point \\( (drumstick, \\cosh drumstick) \\) and also normal to the graph of \\( avalanche=\\sinh blueberries \\) at a point \\( (sailmaker, \\sinh sailmaker) \\).\n[At a point on a graph, the normal line is the perpendicular to the tangent at that point. Also, \\( \\cosh blueberries=\\left(e^{blueberries}+ \\left.e^{-blueberries}\\right) / 2 \\) and \\( \\left.\\sinh blueberries=\\left(e^{blueberries}-e^{-blueberries}\\right) / 2.\\right] \\)",
      "solution": "B-1.\nWe assume that there is such a common normal and obtain a contradiction. This assumption implies\n\\[\n-\\frac{drumstick-sailmaker}{\\cosh drumstick-\\sinh sailmaker}=\\cosh sailmaker=\\sinh drumstick .\n\\]\n\nSince \\( \\cosh blueberries>0 \\) for all real \\( blueberries \\) and \\( \\sinh blueberries>0 \\) only for \\( blueberries>0 \\), (I) implies \\( drumstick>0 \\). Using the fact that \\( \\sinh blueberries<\\cosh blueberries \\) for all \\( blueberries \\) and (I), one obtains\n\\[\n\\sinh sailmaker<\\cosh sailmaker=\\sinh drumstick<\\cosh drumstick .\n\\]\n\nThis, \\( drumstick>0 \\), and the fact that \\( \\cosh blueberries \\) increases for \\( blueberries>0 \\) imply that \\( sailmaker<drumstick \\). Thus the leftmost expression in (I) is negative and cannot equal cosh \\( sailmaker \\). This contradiction shows that no common normal exists."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "verticalaxis",
        "y": "horizontalaxis",
        "a": "negativeroot",
        "c": "startervalue"
      },
      "question": "Problem B-1\nProve or disprove: there is at least one straight line normal to the graph of \\( horizontalaxis=\\cosh verticalaxis \\) at a point \\( (negativeroot, \\cosh negativeroot) \\) and also normal to the graph of \\( horizontalaxis=\\sinh verticalaxis \\) at a point \\( (startervalue, \\sinh startervalue) \\).\n[At a point on a graph, the normal line is the perpendicular to the tangent at that point. Also, \\( \\cosh verticalaxis=\\left(e^{verticalaxis}+\\right. \\) \\( \\left.e^{-verticalaxis}\\right) / 2 \\) and \\( \\left.\\sinh verticalaxis=\\left(e^{verticalaxis}-e^{-verticalaxis}\\right) / 2.\\right] \\)",
      "solution": "B-1.\nWe assume that there is such a common normal and obtain a contradiction. This assumption implies\n\\[\n-\\frac{negativeroot-startervalue}{\\cosh negativeroot-\\sinh startervalue}=\\cosh startervalue=\\sinh negativeroot .\n\\]\n\nSince \\( \\cosh verticalaxis>0 \\) for all real \\( verticalaxis \\) and \\( \\sinh verticalaxis>0 \\) only for \\( verticalaxis>0 \\), (I) implies \\( negativeroot>0 \\). Using the fact that \\( \\sinh verticalaxis<\\cosh verticalaxis \\) for all \\( verticalaxis \\) and (I), one obtains\n\\[\n\\sinh startervalue<\\cosh startervalue=\\sinh negativeroot<\\cosh negativeroot .\n\\]\n\nThis, \\( negativeroot>0 \\), and the fact that \\( \\cosh verticalaxis \\) increases for \\( verticalaxis>0 \\) imply that \\( startervalue<negativeroot \\). Thus the leftmost expression in (I) is negative and cannot equal cosh \\( startervalue \\). This contradiction shows that no common normal exists."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "y": "hjgrksla",
        "a": "frmbzkel",
        "c": "vyqsdton"
      },
      "question": "Problem B-1\nProve or disprove: there is at least one straight line normal to the graph of \\( hjgrksla=\\cosh qzxwvtnp \\) at a point \\( (frmbzkel, \\cosh frmbzkel) \\) and also normal to the graph of \\( hjgrksla=\\sinh qzxwvtnp \\) at a point \\( (vyqsdton, \\sinh vyqsdton) \\).\n[At a point on a graph, the normal line is the perpendicular to the tangent at that point. Also, \\( \\cosh qzxwvtnp=\\left(e^{qzxwvtnp}+\\right. \\) \\( \\left.e^{-qzxwvtnp}\\right) / 2 \\) and \\( \\left.\\sinh qzxwvtnp=\\left(e^{qzxwvtnp}-e^{-qzxwvtnp}\\right) / 2.\\right] \\)",
      "solution": "B-1.\nWe assume that there is such a common normal and obtain a contradiction. This assumption implies\n\\[\n-\\frac{frmbzkel-vyqsdton}{\\cosh frmbzkel-\\sinh vyqsdton}=\\cosh vyqsdton=\\sinh frmbzkel .\n\\]\n\nSince \\( \\cosh qzxwvtnp>0 \\) for all real \\( qzxwvtnp \\) and \\( \\sinh qzxwvtnp>0 \\) only for \\( qzxwvtnp>0 \\), (I) implies \\( frmbzkel>0 \\). Using the fact that \\( \\sinh qzxwvtnp<\\cosh qzxwvtnp \\) for all \\( qzxwvtnp \\) and (I), one obtains\n\\[\n\\sinh vyqsdton<\\cosh vyqsdton=\\sinh frmbzkel<\\cosh frmbzkel .\n\\]\n\nThis, \\( frmbzkel>0 \\), and the fact that \\( \\cosh qzxwvtnp \\) increases for \\( qzxwvtnp>0 \\) imply that \\( vyqsdton<frmbzkel \\). Thus the leftmost expression in (I) is negative and cannot equal cosh \\( vyqsdton \\). This contradiction shows that no common normal exists."
    },
    "kernel_variant": {
      "question": "Let  \n\n C₁ : y = 7 cosh(3x − 5) + 2,  C₂ : y = 7 sinh(3x − 5) + 2.  \n\n(a) Show that at most one straight line can be simultaneously normal to both curves.  \n(b) Determine whether such a common normal actually exists, and justify your answer completely.\n\n———————————————————",
      "solution": "(≈ 100 words)  \nAssume a common normal ℓ meets C₁ at P(a, 7 cosh(3a−5)+2) and C₂ at Q(c, 7 sinh(3c−5)+2).  \nTangent slopes are 21 sinh(3a−5) and 21 cosh(3c−5); hence  \n\n (I) sinh(3a−5)=cosh(3c−5)>0 ⇒ a>5/3 and c<a.  \n\nBecause P,Q lie on ℓ,  \n\n 7[cosh(3a−5)−sinh(3c−5)]/(a−c)=−1/[21 sinh(3a−5)].  \n\nClearing denominators gives  \n\n 21 sinh(3a−5)(cosh(3a−5)−sinh(3c−5))=−(a−c). (II)  \n\nObserve that sinh t < cosh t for all real t, so the bracket in (II) is positive; with sinh(3a−5)>0 we get LHS>0, while a−c>0 makes RHS<0—a contradiction.  \nThe vertical normal at a=5/3 (where sinh term vanishes) cannot match the finite slope on C₂, so no exceptional case arises.  \nTherefore no common normal exists; statement (a) is vacuously satisfied and (b) is negative.\n\n———————————————————",
      "_replacement_note": {
        "replaced_at": "2025-07-05T22:17:12.086503",
        "reason": "Original kernel variant was too easy compared to the original problem"
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}