Question

2cos^2x+5cosx+2<=0 pozhaluistaaaaa

Answer 1

$t=cosx\\2t^2+5t+2 \leq 0\\D=25-4*2*2=9\\t_1=-2,t_2=\frac{-1}{2}$
$-2 \leq cosx \leq -\frac{1}{2}\\-1 \leq cosx \leq 1 \to \\-1 \leq cosx \leq -\frac{1}{2}$
$\frac{2\pi}{3}+2\pi n \leq x \leq \frac{4\pi}{3}+2\pi n, n\in Z$