Prove that cos3x = 4cos3 x -3cosx .
cos3x = cos(2x + x)
= cos2x cos x – sin2xsinx
= (2cos2x - 1)cosx – 2sinxcosxsinx
= 2cos3x - cosx - 2cosx(1 - cos2x)
= 4cos3 x - 3cosx
