2-3cosx-2=0 cos3x+cosx=0 sinx> - cosx≥0 tgx≤

2cos²x-3cosx-2=0   D=9+16=25  √25=5
cosx1=1/4[3+5]=2 не подходит
cosx=1/4[3-5]=-1/2
x=+-2π/3+2πk   k∈Z

cos3x+cosx=0
2cos2xcosx=0
cos2x=0   2x=π/2+2πk   x=π/4+πk   k∈Z
cosx=0  x=π/2+2πk

sinx >-√3/3       (π-arcsin(-√3/3))(-1)^k+2πk >x>arcsin(-√3/3)(-1)^k+2πk
k∈Z

cosx≥0       -π/2+2πk ≤x≤π/2+2πk   k∈Z

<span>tgx≤</span>√3        
 -π/2+πk<x≤π/3+πk   k∈Z