1997 AIME Problem 11

Attempt Problem 11 of the 1997 AIME below, then check your answer against the professionally curated solution 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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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?

Answer: 241
Concepts:trigonometric identitytelescoping
Difficulty rating: 2710
Small Hint:

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

Big Hint:

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

Solution:

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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