cos(2x+x)
=cos (2x)cosx-sin(2x)sinx
=(1-2sin^2x)cosx-2sinxcosx*cos x
=cos x-2 sin ^2 x cos x-2 sin^2 xcos x
=cos x-4sin^2 x cos x
=cos x {1-4(1- cos ^2 x)}
=4 cos ^3x -3 cos x
