Question

Help with a trig proof please?

`cos(3x)=cosx-3sin^2xcosx`

Answer 1

`LHS=cos3x`

`=cos(x+2x)`

`=cosxcos2x-sinxsin2x`

`=cosx(1-2sin^2x)-sinx*2sinxcosx`

`=cosx-2sin^2xcosx-sinx*2sinxcosx`

`=cosx-3sin^2xcosx=RHS`