1997 AIME 第 11 题

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11.

x=n=144cosnn=144sinnx = \frac{\displaystyle\sum_{n=1}^{44} \cos n^\circ}{\displaystyle\sum_{n=1}^{44} \sin n^\circ}\text{。}求不超过 100x100x 的最大整数。

Let x=n=144cosnn=144sinn.x = \frac{\displaystyle\sum_{n=1}^{44} \cos n^\circ}{\displaystyle\sum_{n=1}^{44} \sin n^\circ}. What is the greatest integer that does not exceed 100x?100x?

答案:241
知识点:三角恒等式裂项相消
难度评级:2710
小提示:

分子分母同乘 2sin122\sin\frac{1}{2}^\circ,并用积化和差公式裂项求和

Multiply numerator and denominator by 2sin122\sin\frac{1}{2}^\circ and telescope using product-to-sum identities

大提示:

和差化积会把结果化为 cot22.5\cot 22.5^\circ,再用半角公式精确求值

Sum-to-product turns the result into cot22.5,\cot 22.5^\circ, which the half-angle formula evaluates exactly

解答:

分子分母同乘 2sin122\sin\frac{1}{2}^\circ。由于 2cosnsin122\cos n^\circ \sin\frac{1}{2}^\circ =sin(n+12)= \sin\left(n + \frac{1}{2}\right)^\circ sin(n12)- \sin\left(n - \frac{1}{2}\right)^\circ,且 2sinnsin122\sin n^\circ \sin\frac{1}{2}^\circ =cos(n12)= \cos\left(n - \frac{1}{2}\right)^\circ cos(n+12)- \cos\left(n + \frac{1}{2}\right)^\circ,两边的求和都会裂项相消:x=sin44.5sin0.5cos0.5cos44.5=2cos22.5sin222sin22.5sin22=cot22.5 \begin{aligned} x &= \frac{\sin 44.5^\circ - \sin 0.5^\circ}{\cos 0.5^\circ - \cos 44.5^\circ} \\ &= \frac{2\cos 22.5^\circ \sin 22^\circ}{2\sin 22.5^\circ \sin 22^\circ} \\ &= \cot 22.5^\circ \end{aligned}\text{。}最后一步使用了和差化积公式。

由半角公式,cot22.5=1+cos45sin45=2+1\cot 22.5^\circ = \frac{1 + \cos 45^\circ}{\sin 45^\circ} = \sqrt{2} + 1。因为 1.412<2<1.4221.41^2 \lt 2 \lt 1.42^2,所以 241<1002+100<242241 \lt 100\sqrt{2} + 100 \lt 242。因此,不超过 100x100x 的最大整数是 241241

Multiply numerator and denominator by 2sin12.2\sin\frac{1}{2}^\circ. Since 2cosnsin122\cos n^\circ \sin\frac{1}{2}^\circ =sin(n+12)= \sin\left(n + \frac{1}{2}\right)^\circ sin(n12)- \sin\left(n - \frac{1}{2}\right)^\circ and 2sinnsin122\sin n^\circ \sin\frac{1}{2}^\circ =cos(n12)= \cos\left(n - \frac{1}{2}\right)^\circ cos(n+12),- \cos\left(n + \frac{1}{2}\right)^\circ, both sums telescope: x=sin44.5sin0.5cos0.5cos44.5=2cos22.5sin222sin22.5sin22=cot22.5, \begin{aligned} x &= \frac{\sin 44.5^\circ - \sin 0.5^\circ}{\cos 0.5^\circ - \cos 44.5^\circ} \\ &= \frac{2\cos 22.5^\circ \sin 22^\circ}{2\sin 22.5^\circ \sin 22^\circ} \\ &= \cot 22.5^\circ, \end{aligned} using the sum-to-product identities in the last step.

By the half-angle formula, cot22.5=1+cos45sin45=2+1.\cot 22.5^\circ = \frac{1 + \cos 45^\circ}{\sin 45^\circ} = \sqrt{2} + 1. Since 1.412<2<1.422,1.41^2 \lt 2 \lt 1.42^2, we have 241<1002+100<242.241 \lt 100\sqrt{2} + 100 \lt 242. Therefore the greatest integer not exceeding 100x100x is 241.241.

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