Question

How do find the derivative of `y = cos^2(x)`?

Answer 1

First of all `y=cos^2x=(cosx)^2`

Hence

`y'=2cosx*(cosx)'=2cosx*(-sinx)=-2cosx*sinx=-sin2x`

Another way is

`y=cos^2x=1/2(1+cos2x)`

Hence

`y'=1/2*(-sin2x *(2x)')=-sin2x`