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
tgx≤√3
-π/2+πk