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**[SOLVED] Concavity of Trig Functions**

I'm working out a problem, but I'm stuck at a spot.

The original equation is y = 2 cos x + sin 2x, and yes I did the second derivative, which came out to be

cos x (4 sin x + 1)

when i equate it to zero, i get

cos x = 0 and 4 sin x + 1 = 0

(Let's just say the domain was defined from 0 to 2 pi)

For SOME reason, I keep trying to evaluate x when 4 sin x + 1 = 0

by using inverse sin ....so x = inverse sin (-0.25)

My calculator is in radians, but it still gives me an answer of -0.25. I used a graphing calculator, and the answer SHOULD be 3.39. What am I doing wrong?