2(cos³x sinx-sin³x cosx)=2sin2x cos2x
2 sinx cosx (cos^2 x - sin^2 x)= 2sin2x cos2x
2 sinx cosx 【（1+cos2x)/2 - (1-cos2x)/2】=2 sin2x cos2x
sin2x (1+cos2x-1+cos2x)/2 = 2 sin2x cos2x
sin2x cos2x = 2 sin2x cos2x
2 sin2x cos2x - sin2x cos2x = 0
sin2x cos2x = 0
sin2x= 0 hoặc cos2x=0
2x = k*pi hoặc 2x= pi/2 + k*pi
x = k*pi/2 hoặc x = pi/4 + k*pi/2