Question

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

Answer 1

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