1997 AIME Problem 11

Below is the professionally curated solution for Problem 11 of the 1997 AIME, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1997 AIME solutions, or check the answer key.

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Concepts:trigonometric identitytelescoping

Difficulty rating: 2710

11.

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?

Solution:

Multiply numerator and denominator by 2sin12.2\sin\frac{1}{2}^\circ. Since 2cosnsin12=sin(n+12)sin(n12)2\cos n^\circ \sin\frac{1}{2}^\circ = \sin\left(n + \frac{1}{2}\right)^\circ - \sin\left(n - \frac{1}{2}\right)^\circ and 2sinnsin12=cos(n12)cos(n+12),2\sin n^\circ \sin\frac{1}{2}^\circ = \cos\left(n - \frac{1}{2}\right)^\circ - \cos\left(n + \frac{1}{2}\right)^\circ, both sums telescope: x=sin44.5sin0.5cos0.5cos44.5=2cos22.5sin222sin22.5sin22=cot22.5,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, 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. Hence 100x=1002+100=241.42,100x = 100\sqrt{2} + 100 = 241.42\ldots, and the greatest integer not exceeding it is 241.241.

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