2cos^3x-cos^2x-cosx решите уравнение

сosx(2cos²x-cosx-1)=0
cosx=0⇔x=π/2+πn
2cos²x-cosx-1=0
cosx=a
2a²-a-1=0
D=1+8=9
a1=(1+3)/4=1⇒cosx=1⇒x=2πn
a2=(1-3)/4=-1/2⇒cosx=-1/2⇒x=+-2π/3+2π