1997 AIME 第 11 题

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11.

令 x=∑n=144cos⁡n∘∑n=144sin⁡n∘。x = \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=144cos⁡n∘∑n=144sin⁡n∘.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 2sin⁡12∘2\sin\frac{1}{2}^\circ,并用积化和差公式裂项求和

Multiply numerator and denominator by 2sin⁡12∘2\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 cot⁡22.5∘\cot 22.5^\circ,再用半角公式精确求值

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

解答:

分子分母同乘 2sin⁡12∘2\sin\frac{1}{2}^\circ。由于 2cos⁡n∘sin⁡12∘2\cos n^\circ \sin\frac{1}{2}^\circ =sin⁡(n+12)∘= \sin\left(n + \frac{1}{2}\right)^\circ −sin⁡(n−12)∘- \sin\left(n - \frac{1}{2}\right)^\circ,且 2sin⁡n∘sin⁡12∘2\sin n^\circ \sin\frac{1}{2}^\circ =cos⁡(n−12)∘= \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=sin⁡44.5∘−sin⁡0.5∘cos⁡0.5∘−cos⁡44.5∘=2cos⁡22.5∘sin⁡22∘2sin⁡22.5∘sin⁡22∘=cot⁡22.5∘。 \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,cot⁡22.5∘=1+cos⁡45∘sin⁡45∘=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 2sin⁡12∘.2\sin\frac{1}{2}^\circ. Since 2cos⁡n∘sin⁡12∘2\cos n^\circ \sin\frac{1}{2}^\circ =sin⁡(n+12)∘= \sin\left(n + \frac{1}{2}\right)^\circ −sin⁡(n−12)∘- \sin\left(n - \frac{1}{2}\right)^\circ and 2sin⁡n∘sin⁡12∘2\sin n^\circ \sin\frac{1}{2}^\circ =cos⁡(n−12)∘= \cos\left(n - \frac{1}{2}\right)^\circ −cos⁡(n+12)∘,- \cos\left(n + \frac{1}{2}\right)^\circ, both sums telescope: x=sin⁡44.5∘−sin⁡0.5∘cos⁡0.5∘−cos⁡44.5∘=2cos⁡22.5∘sin⁡22∘2sin⁡22.5∘sin⁡22∘=cot⁡22.5∘, \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, cot⁡22.5∘=1+cos⁡45∘sin⁡45∘=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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