Question

What is the derivative of `f(x)=cosx*cos2x`?

Answer 1

`-sinxcos2x-2sin2xcosx`

Explanation:

Use the Product Rule.

Also, find the derivatives of the two terms first.

`d/dx[cosx]=-sinx`
`d/dx[cos2x]=-2sin2x`

`d/dx[cosx*cos2x]=-sinxcos2x-2sin2xcosx`