{
  "index": "1973-B-6",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "B-6. On the domain \\( 0 \\leqq \\theta \\leqq 2 \\pi \\) :\n(a) Prove that \\( \\sin ^{2} \\theta \\cdot \\sin (2 \\theta) \\) takes its maximum at \\( \\pi / 3 \\) and \\( 4 \\pi / 3 \\) (and hence its minimum at \\( 2 \\pi / 3 \\) and \\( 5 \\pi / 3 \\) ).\n(b) Show that\n\\[\n\\left|\\sin ^{2} \\theta\\left\\{\\sin ^{3}(2 \\theta) \\cdot \\sin 3(4 \\theta) \\cdots \\sin ^{3}\\left(2^{n-1} \\theta\\right)\\right\\} \\sin \\left(2^{n} \\theta\\right)\\right|\n\\]\ntakes its maximum at \\( \\theta=\\pi / 3 \\). (The maximum may also be attained at other points.)\n(c) Derive the inequality:\n\\[\n\\sin ^{2} \\theta \\cdot \\sin ^{2}(2 \\theta) \\cdot \\sin ^{2}(4 \\theta) \\cdots \\sin ^{2}\\left(2^{n} \\theta\\right) \\leqq(3 / 4)^{n}\n\\]",
  "solution": "B-6. (a) Simple calculus.\n(b) By induction: The case \\( n=1 \\) is just (a).\n\nNow the ratio of the expression for \\( n+1 \\) to the expression for \\( n \\) is equal to:\n\\[\n\\left|\\sin ^{2} 2^{n} \\theta \\cdot \\sin 2^{n+1} \\theta\\right|\n\\]\n\nSince \\( \\theta=\\pi / 3 \\) gives \\( 2^{n} \\theta \\equiv 2 \\pi / 3 \\) or \\( 4 \\pi / 3(\\bmod 2 \\pi) \\), this ratio is maximized at \\( \\theta=\\pi / 3 \\), and by induction, then, the whole expression is maximized.\n(c) Set \\( \\theta=\\pi / 3 \\), and observe that the expression in part (b) is then exactly equal to \\( (3 / 4)^{3 n / 2} \\); its \\( 2 / 3 \\) power is thus equal to \\( (3 / 4)^{n} \\). That is the maximum; in general the \\( 2 / 3 \\) power of the expression in (b) is \\( \\leqq(3 / 4)^{n} \\). To get from that to the expression in (c), we would increase the powers of the end factors \\( \\sin \\theta \\) and \\( \\sin 2^{n} \\theta \\); this can only decrease the product, since \\( |\\sin \\theta| \\leqq 1 \\).",
  "vars": [
    "\\\\theta"
  ],
  "params": [
    "n"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "\\theta": "angletheta",
        "n": "indexcount"
      },
      "question": "B-6. On the domain \\( 0 \\leqq angletheta \\leqq 2 \\pi \\) :\n(a) Prove that \\( \\sin ^{2} angletheta \\cdot \\sin (2 angletheta) \\) takes its maximum at \\( \\pi / 3 \\) and \\( 4 \\pi / 3 \\) (and hence its minimum at \\( 2 \\pi / 3 \\) and \\( 5 \\pi / 3 \\) ).\n(b) Show that\n\\[\n\\left|\\sin ^{2} angletheta\\left\\{\\sin ^{3}(2 angletheta) \\cdot \\sin 3(4 angletheta) \\cdots \\sin ^{3}\\left(2^{indexcount-1} angletheta\\right)\\right\\} \\sin \\left(2^{indexcount} angletheta\\right)\\right|\n\\]\ntakes its maximum at \\( angletheta=\\pi / 3 \\). (The maximum may also be attained at other points.)\n(c) Derive the inequality:\n\\[\n\\sin ^{2} angletheta \\cdot \\sin ^{2}(2 angletheta) \\cdot \\sin ^{2}(4 angletheta) \\cdots \\sin ^{2}\\left(2^{indexcount} angletheta\\right) \\leqq(3 / 4)^{indexcount}\n\\]",
      "solution": "B-6. (a) Simple calculus.\n(b) By induction: The case \\( indexcount=1 \\) is just (a).\n\nNow the ratio of the expression for \\( indexcount+1 \\) to the expression for \\( indexcount \\) is equal to:\n\\[\n\\left|\\sin ^{2} 2^{indexcount} angletheta \\cdot \\sin 2^{indexcount+1} angletheta\\right|\n\\]\n\nSince \\( angletheta=\\pi / 3 \\) gives \\( 2^{indexcount} angletheta \\equiv 2 \\pi / 3 \\) or \\( 4 \\pi / 3(\\bmod 2 \\pi) \\), this ratio is maximized at \\( angletheta=\\pi / 3 \\), and by induction, then, the whole expression is maximized.\n(c) Set \\( angletheta=\\pi / 3 \\), and observe that the expression in part (b) is then exactly equal to \\( (3 / 4)^{3 indexcount / 2} \\); its \\( 2 / 3 \\) power is thus equal to \\( (3 / 4)^{indexcount} \\). That is the maximum; in general the \\( 2 / 3 \\) power of the expression in (b) is \\( \\leqq(3 / 4)^{indexcount} \\). To get from that to the expression in (c), we would increase the powers of the end factors \\( \\sin angletheta \\) and \\( \\sin 2^{indexcount} angletheta \\); this can only decrease the product, since \\( |\\sin angletheta| \\leqq 1 \\)."
    },
    "descriptive_long_confusing": {
      "map": {
        "\\theta": "buttercup",
        "n": "caterpillar"
      },
      "question": "B-6. On the domain \\( 0 \\leqq buttercup \\leqq 2 \\pi \\) :\n(a) Prove that \\( \\sin ^{2} buttercup \\cdot \\sin (2 buttercup) \\) takes its maximum at \\( \\pi / 3 \\) and \\( 4 \\pi / 3 \\) (and hence its minimum at \\( 2 \\pi / 3 \\) and \\( 5 \\pi / 3 \\) ).\n(b) Show that\n\\[\n\\left|\\sin ^{2} buttercup\\left\\{\\sin ^{3}(2 buttercup) \\cdot \\sin 3(4 buttercup) \\cdots \\sin ^{3}\\left(2^{caterpillar-1} buttercup\\right)\\right\\} \\sin \\left(2^{caterpillar} buttercup\\right)\\right|\n\\]\ntakes its maximum at \\( buttercup=\\pi / 3 \\). (The maximum may also be attained at other points.)\n(c) Derive the inequality:\n\\[\n\\sin ^{2} buttercup \\cdot \\sin ^{2}(2 buttercup) \\cdot \\sin ^{2}(4 buttercup) \\cdots \\sin ^{2}\\left(2^{caterpillar} buttercup\\right) \\leqq(3 / 4)^{caterpillar}\n\\]",
      "solution": "B-6. (a) Simple calculus.\n(b) By induction: The case \\( caterpillar=1 \\) is just (a).\n\nNow the ratio of the expression for \\( caterpillar+1 \\) to the expression for \\( caterpillar \\) is equal to:\n\\[\n\\left|\\sin ^{2} 2^{caterpillar} buttercup \\cdot \\sin 2^{caterpillar+1} buttercup\\right|\n\\]\n\nSince \\( buttercup=\\pi / 3 \\) gives \\( 2^{caterpillar} buttercup \\equiv 2 \\pi / 3 \\) or \\( 4 \\pi / 3(\\bmod 2 \\pi) \\), this ratio is maximized at \\( buttercup=\\pi / 3 \\), and by induction, then, the whole expression is maximized.\n(c) Set \\( buttercup=\\pi / 3 \\), and observe that the expression in part (b) is then exactly equal to \\( (3 / 4)^{3 caterpillar / 2} \\); its \\( 2 / 3 \\) power is thus equal to \\( (3 / 4)^{caterpillar} \\). That is the maximum; in general the \\( 2 / 3 \\) power of the expression in (b) is \\( \\leqq(3 / 4)^{caterpillar} \\). To get from that to the expression in (c), we would increase the powers of the end factors \\( \\sin buttercup \\) and \\( \\sin 2^{caterpillar} buttercup \\); this can only decrease the product, since \\( |\\sin buttercup| \\leqq 1 \\)."
    },
    "descriptive_long_misleading": {
      "map": {
        "\\theta": "straightline",
        "n": "uncountable"
      },
      "question": "B-6. On the domain \\( 0 \\leqq straightline \\leqq 2 \\pi \\) :\n(a) Prove that \\( \\sin ^{2} straightline \\cdot \\sin (2 straightline) \\) takes its maximum at \\( \\pi / 3 \\) and \\( 4 \\pi / 3 \\) (and hence its minimum at \\( 2 \\pi / 3 \\) and \\( 5 \\pi / 3 \\) ).\n(b) Show that\n\\[\n\\left|\\sin ^{2} straightline\\left\\{\\sin ^{3}(2 straightline) \\cdot \\sin 3(4 straightline) \\cdots \\sin ^{3}\\left(2^{uncountable-1} straightline\\right)\\right\\} \\sin \\left(2^{uncountable} straightline\\right)\\right|\n\\]\n takes its maximum at \\( straightline=\\pi / 3 \\). (The maximum may also be attained at other points.)\n(c) Derive the inequality:\n\\[\n\\sin ^{2} straightline \\cdot \\sin ^{2}(2 straightline) \\cdot \\sin ^{2}(4 straightline) \\cdots \\sin ^{2}\\left(2^{uncountable} straightline\\right) \\leqq(3 / 4)^{uncountable}\n\\]",
      "solution": "B-6. (a) Simple calculus.\n(b) By induction: The case \\( uncountable=1 \\) is just (a).\n\nNow the ratio of the expression for \\( uncountable+1 \\) to the expression for \\( uncountable \\) is equal to:\n\\[\n\\left|\\sin ^{2} 2^{uncountable} straightline \\cdot \\sin 2^{uncountable+1} straightline\\right|\n\\]\n\nSince \\( straightline=\\pi / 3 \\) gives \\( 2^{uncountable} straightline \\equiv 2 \\pi / 3 \\) or \\( 4 \\pi / 3(\\bmod 2 \\pi) \\), this ratio is maximized at \\( straightline=\\pi / 3 \\), and by induction, then, the whole expression is maximized.\n(c) Set \\( straightline=\\pi / 3 \\), and observe that the expression in part (b) is then exactly equal to \\( (3 / 4)^{3 uncountable / 2} \\); its \\( 2 / 3 \\) power is thus equal to \\( (3 / 4)^{uncountable} \\). That is the maximum; in general the \\( 2 / 3 \\) power of the expression in (b) is \\( \\leqq(3 / 4)^{uncountable} \\). To get from that to the expression in (c), we would increase the powers of the end factors \\( \\sin straightline \\) and \\( \\sin 2^{uncountable} straightline \\); this can only decrease the product, since \\( |\\sin straightline| \\leqq 1 \\)."
    },
    "garbled_string": {
      "map": {
        "\\theta": "qzxwvtnp",
        "n": "hjgrksla"
      },
      "question": "B-6. On the domain \\( 0 \\leqq qzxwvtnp \\leqq 2 \\pi \\) :\n(a) Prove that \\( \\sin ^{2} qzxwvtnp \\cdot \\sin (2 qzxwvtnp) \\) takes its maximum at \\( \\pi / 3 \\) and \\( 4 \\pi / 3 \\) (and hence its minimum at \\( 2 \\pi / 3 \\) and \\( 5 \\pi / 3 \\) ).\n(b) Show that\n\\[\n\\left|\\sin ^{2} qzxwvtnp\\left\\{\\sin ^{3}(2 qzxwvtnp) \\cdot \\sin 3(4 qzxwvtnp) \\cdots \\sin ^{3}\\left(2^{hjgrksla-1} qzxwvtnp\\right)\\right\\} \\sin \\left(2^{hjgrksla} qzxwvtnp\\right)\\right|\n\\]\n takes its maximum at \\( qzxwvtnp=\\pi / 3 \\). (The maximum may also be attained at other points.)\n(c) Derive the inequality:\n\\[\n\\sin ^{2} qzxwvtnp \\cdot \\sin ^{2}(2 qzxwvtnp) \\cdot \\sin ^{2}(4 qzxwvtnp) \\cdots \\sin ^{2}\\left(2^{hjgrksla} qzxwvtnp\\right) \\leqq(3 / 4)^{hjgrksla}\n\\]",
      "solution": "B-6. (a) Simple calculus.\n(b) By induction: The case \\( hjgrksla=1 \\) is just (a).\n\nNow the ratio of the expression for \\( hjgrksla+1 \\) to the expression for \\( hjgrksla \\) is equal to:\n\\[\n\\left|\\sin ^{2} 2^{hjgrksla} qzxwvtnp \\cdot \\sin 2^{hjgrksla+1} qzxwvtnp\\right|\n\\]\n\nSince \\( qzxwvtnp=\\pi / 3 \\) gives \\( 2^{hjgrksla} qzxwvtnp \\equiv 2 \\pi / 3 \\) or \\( 4 \\pi / 3(\\bmod 2 \\pi) \\), this ratio is maximized at \\( qzxwvtnp=\\pi / 3 \\), and by induction, then, the whole expression is maximized.\n(c) Set \\( qzxwvtnp=\\pi / 3 \\), and observe that the expression in part (b) is then exactly equal to \\( (3 / 4)^{3 hjgrksla / 2} \\); its \\( 2 / 3 \\) power is thus equal to \\( (3 / 4)^{hjgrksla} \\). That is the maximum; in general the \\( 2 / 3 \\) power of the expression in (b) is \\( \\leqq(3 / 4)^{hjgrksla} \\). To get from that to the expression in (c), we would increase the powers of the end factors \\( \\sin qzxwvtnp \\) and \\( \\sin 2^{hjgrksla} qzxwvtnp \\); this can only decrease the product, since \\( |\\sin qzxwvtnp| \\leqq 1 \\)."
    },
    "kernel_variant": {
      "question": "Let n be a non-negative integer and let 0\\le \\theta\\le 2\\pi.  Define\n\nP_{n}(\\theta)=\\;\\sin ^{2}\\!\\theta\\;\\Bigl[\\,\\sin ^{3}(2\\theta)\\,\\sin ^{3}(2^{2}\\theta)\\cdots \\sin ^{3}(2^{n-1}\\theta)\\Bigr] \\;\\sin (2^{n}\\theta).\n\n(a)  Put f(\\theta)=\\sin ^{2}\\theta\\,\\sin (2\\theta).  Show that f attains its maximum value 3\\sqrt 3\\,/\\,8 and that this maximum is reached precisely for\n        \\theta \\equiv \\pi/3 \\text{ or } \\theta \\equiv 4\\pi/3 \\pmod {2\\pi}.\n\n(b)  Prove that for every integer n\\ge 1 and every real \\theta\n            |P_{n}(\\theta)|\\;\\le \\;(3\\sqrt 3/8)^{\\,n},\n     and determine all \\theta for which equality holds.\n\n(c)  Deduce that for every real \\theta and every integer n\\ge 0\n            \\sin ^{2}\\theta\\;\\sin ^{2}(2\\theta)\\;\\sin ^{2}(2^{2}\\theta)\\;\\cdots\\;\\sin ^{2}(2^{n}\\theta)\\;\\le\\;(3/4)^{\\,n}.",
      "solution": "Throughout write  M:=3\\sqrt 3/8.\n\n-------------------------------------------------\n(a)  The maximum of f(\\theta)=\\sin ^{2}\\theta\\,\\sin (2\\theta).\n\nUse the identity  \\sin 2\\theta=2\\sin \\theta\\cos \\theta:\n     f(\\theta)=2\\sin ^{3}\\theta\\cos \\theta.\n\nDifferentiate:\n     f'(\\theta)=2(3\\sin ^{2}\\theta\\cos ^{2}\\theta-\\sin ^{4}\\theta)\n               =2\\sin ^{2}\\theta\\,(3\\cos ^{2}\\theta-\\sin ^{2}\\theta)\n               =2\\sin ^{2}\\theta\\,(3-4\\sin ^{2}\\theta).\n\nThus f'(\\theta)=0 when \\sin \\theta=0   (\\theta=0,\\pi ,2\\pi) or when \\sin ^{2}\\theta=3/4 (\\theta=\\pi/3,2\\pi/3,4\\pi/3,5\\pi/3).\n\nEvaluating f at these critical points gives\n     f(0)=f(\\pi)=f(2\\pi)=0,\n     f(\\pi/3)=f(4\\pi/3)=+M,\n     f(2\\pi/3)=f(5\\pi/3)=-M.\nBecause |f(\\theta)|\\le 2 for all \\theta, the value M is the global maximum, reached exactly when \\theta\\equiv\\pi/3 or 4\\pi/3 (mod 2\\pi).\n\n-------------------------------------------------\n(b)  Bounding |P_{n}(\\theta)| and the equality cases.\n\nWrite\n     P_{n}(\\theta)=\\sin ^{2}\\theta\\;\\prod _{k=1}^{n-1}\\bigl[\\sin ^{2}(2^{k}\\theta)\\,\\sin (2^{k}\\theta)\\bigr] \\;\\sin (2^{n}\\theta)\n                  =\\prod _{j=0}^{n-1} f\\bigl(2^{j}\\theta\\bigr).               (\\*)\n\nTaking absolute values and using part (a),\n     |P_{n}(\\theta)|\\;=\\;\\prod _{j=0}^{n-1}|f(2^{j}\\theta)|\\;\\le\\;M^{n}.      (1)\n\nEquality in (1) requires |f(2^{j}\\theta)|=M for every j=0,1,\\dots ,n-1; equivalently\n                   2^{j}\\theta\\equiv \\pi/3,\\;2\\pi/3,\\;4\\pi/3,\\;5\\pi/3 \\pmod{2\\pi}.      (2)\nBecause multiplication by 2 modulo \\pi simply interchanges the two residue classes \\pi/3 and 2\\pi/3, condition (2) is satisfied for every j iff\n                   \\theta\\equiv \\pi/3 \\text{ or } 2\\pi/3 \\pmod{\\pi}.               (3)\nConversely, (3) indeed forces (2).  Hence\n     |P_{n}(\\theta)|=M^{n}\\quad\\Longleftrightarrow\\quad\\theta=\\pi/3+k\\pi\\text{ or }\\theta=2\\pi/3+k\\pi\\;(k\\in\\mathbb Z).\n(The sign of P_{n}(\\theta) may vary, but its magnitude is M^{n}.)\n\n-------------------------------------------------\n(c)  Bounding the product of even powers.\n\nPut  A_{k}:=\\sin ^{2}(2^{k}\\theta)\\;(0\\le k\\le n).  From (\\*) one has\n     |P_{n}|=A_{0}^{1}\\,A_{1}^{3/2}\\,A_{2}^{3/2}\\cdots A_{n-1}^{3/2}\\,A_{n}^{1/2}.\nTaking the power 2/3 gives\n     |P_{n}|^{2/3}=A_{0}^{2/3}\\,A_{1}\\,A_{2}\\cdots A_{n-1}\\,A_{n}^{1/3}.      (4)\nNow\n     Q_{n}:=\\prod_{k=0}^{n}A_{k}=A_{0}\\,A_{1}\\cdots A_{n-1}\\,A_{n}\n          =\\bigl(|P_{n}|^{2/3}\\bigr)\\;A_{0}^{1/3}\\,A_{n}^{2/3}.                 (5)\nSince 0\\le A_{k}\\le 1, the extra factors in (5) satisfy A_{0}^{1/3}A_{n}^{2/3}\\le 1, whence\n     Q_{n}\\le |P_{n}|^{2/3}.                                                   (6)\nUsing (1), we obtain\n     Q_{n}\\le \\bigl(M^{n}\\bigr)^{2/3}=(M^{2/3})^{n}.\nA direct calculation gives M^{2/3}=(3\\sqrt 3/8)^{2/3}=3/4, so\n     \\boxed{\\;\\sin ^{2}\\theta\\,\\sin ^{2}(2\\theta)\\,\\cdots\\,\\sin ^{2}(2^{n}\\theta)\\le (3/4)^{\\,n}\\;}.  \n\nEquality in (6) would require A_{0}=A_{n}=1, which is impossible when n\\ge 1 because A_{k}=\\sin ^{2}(\\cdot)\\le 1 and the equality |f(\\cdot)|=M forces A_{k}=3/4.  Hence the inequality in part (c) is strict for every n\\ge 1.\n\nThe problem is completely solved.",
      "_meta": {
        "core_steps": [
          "Maximize f(θ)=sin^2θ·sin(2θ) on [0,2π] via one–variable calculus (gives θ*=π/3 mod π).",
          "Write E_n(θ)=sin^2θ·[∏_{i=1}^{n-1}sin^3(2^iθ)]·sin(2^nθ) and note E_{n+1}/E_n = f(2^nθ).",
          "Induct: because 2^nθ* attains f’s maximum for every n, E_n(θ) is maximized at θ*.",
          "Evaluate E_n(θ*) to obtain exact maximum, take 2/3-power, and enlarge the end exponents to deduce ∏_{k=0}^{n}sin^2(2^kθ) ≤ (3/4)^n."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Exponent on the leading sine factor and, consequently, (exponent+1) on each interior factor; changing 2→a (with a>0) and 3→a+1 leaves ratio E_{n+1}/E_n = sin^a x·sin(2x) unchanged in form and the whole proof goes through verbatim.",
            "original": "2 (leading factor), 3 (interior factors)"
          },
          "slot2": {
            "description": "Specific maximizer θ*=π/3 (and 4π/3); any value that maximizes sin^aθ·sin(2θ) and is carried by doubling into the same maximizing set for all powers would serve.",
            "original": "π/3"
          },
          "slot3": {
            "description": "Numerical bound (3/4)^n that appears after evaluating at θ*; this constant changes automatically if slot1 or slot2 is altered, but the inequality’s structure stays identical.",
            "original": "(3/4)^n"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}