Question

What is the derivative of `k(x)=sin x cos x`?

Answer 1

You can use the Product Rule:
if: `k(x)=f(x)g(x)`
`k'(x)=f'(x)g(x)+f(x)g'(x)`
In your case:
`k'(x)=cos(x)cos(x)+sin(x)(-sin(x))=`
`=cos^2(x)-sin^2(x)=cos(2x)`