Question

Cos2x<0

Pomogite......

Answer 1

$Cos2x<0\\frac{pi }{2}+2pi n<2x<frac{3pi }{2}+2pi n,nin z\\frac{pi }{4}+pi n<x<frac{3pi }{4}+pi n,nin z\\xin(frac{pi }{4}+pi n;frac{3pi }{4}+pi n),nin z$

Answer 2

$cos(2x) < 0 \ - frac{3pi}{2} + 2pi : n < 2x < - frac{pi}{2} + 2pi : n \ - frac{3pi}{4} + pi : n < x < - frac{pi}{4} + pi : n$