A geometry problem by Jose Sacramento

Geometry Level 4

Evaluate the sum of all the solutions in the interval [ 0 , π ] [0,\pi ] for the following equation.

2 cos x + 3 sin x = 2 2 \cos x + 3\sin x= 2


The answer is 1.96.

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2 solutions

Ayush G Rai
Oct 31, 2016

Chew-Seong Cheong
Oct 31, 2016

2 cos x + 3 sin x = 2 Divide both sides by cos x 2 + 3 tan x = 2 sec x Square both sides 4 + 12 tan x + 9 tan 2 x = 4 sec 2 x 4 + 12 tan x + 9 tan 2 x = 4 + 4 tan 2 x 12 tan x + 5 tan 2 x = 0 tan x ( 5 tan x + 12 ) = 0 \begin{aligned} 2\cos x + 3 \sin x & = 2 & \small {\color{#3D99F6}\text{Divide both sides by }\cos x} \\ 2 + 3 \tan x & = 2 \sec x & \small {\color{#3D99F6}\text{Square both sides}} \\ 4 + 12 \tan x + 9 \tan^2 x & = 4 \sec^2 x \\ 4 + 12 \tan x + 9 \tan^2 x & = 4 + 4\tan^2 x \\ 12 \tan x + 5 \tan^2 x & = 0 \\ \tan x (5\tan x + 12) & = 0 \end{aligned}

tan x = { 0 x = 0 12 5 x 1.9656 \implies \tan x = \begin{cases} 0 & \implies x = 0 \\ -\frac {12}5 & \implies x \approx 1.9656 \end{cases}

The sum of solutions is therefore 1.96 \approx \boxed{1.96} .

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