1991 AIME Problem 9

Attempt Problem 9 of the 1991 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 1991 AIME solutions, or check the answer key.

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9.

Suppose that sec⁡x+tan⁡x=227\sec x+\tan x=\frac{22}{7} and that csc⁡x+cot⁡x=mn,\csc x+\cot x=\frac{m}{n}, where mn\frac{m}{n} is in lowest terms. Find m+n.m+n.

Answer: 44
Concepts:trigonometric identityalgebraic manipulationfraction
Difficulty rating: 2020
Small Hint:

If u=sec⁡x+tan⁡xu=\sec x+\tan x, then sec⁡x−tan⁡x=1u\sec x-\tan x=\frac{1}{u}

Big Hint:

Rewrite csc⁡x+cot⁡x\csc x+\cot x as sec⁡x+1tan⁡x\frac{\sec x+1}{\tan x}

Solution:

Let u=sec⁡x+tan⁡x=227.u=\sec x+\tan x=\frac{22}{7}. Since sec⁡x+tan⁡x\sec x+\tan x and sec⁡x−tan⁡x\sec x-\tan x have product 1,1, sec⁡x=u+u−12,tan⁡x=u−u−12.\begin{aligned}\sec x&=\frac{u+u^{-1}}2,\\\tan x&=\frac{u-u^{-1}}2.\end{aligned} Also csc⁡x+cot⁡x=1+cos⁡xsin⁡x=sec⁡x+1tan⁡x=u+1u−1.\begin{aligned}\csc x+\cot x&=\frac{1+\cos x}{\sin x}\\&=\frac{\sec x+1}{\tan x}\\&=\frac{u+1}{u-1}.\end{aligned} Substituting u=227u=\frac{22}{7} gives 2915.\frac{29}{15}. Thus m+n=29+15=44.m+n=29+15=44.

Problem 8#8
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