trig
posted by Rin .
By expressing 3x as (2x + x) prove that
cos3x = 4cos^3x  3cosx

cos(3x)
= cos(2x + x)
= cos2xcosx  sin2xsinx
= (2cos^2 x 1)cosx  (2sinxcosx)sinx
= 2cos^3 x  cosx  2sin^2 x (cosx)
= 2 cos^3x  cosx  2(1  cos^2 x)(cosx)
= 2 cos^3x  cosx  2cosx + 2cos^3x
= 4cos^3x  3cosx
= RS
Nice!