Cosx(-1-2cosx)=0 на промежутке [-3пи/2 ; 0]
COSX(-1)-2COS^2X=0
COSX-2COS^2X=0
COSX-2(1+COS2x/2)=0
cosx-1+cos2x=0
cosx+cos2x=1
cosx+(cos^2x-sin^2x)=cos^2x+sin^2x
cosx+cos^2x-sin^2x-cos^2x+sin^2x=0
cosx=0