Hi, for the life of me I cannot figure this one out.

Find the derivative of $\displaystyle sin^4 \sqrt{u}$.

This is what I tried to do:

$\displaystyle sin^4 \sqrt{u} = (sin\sqrt{u})^4$

$\displaystyle =4(sin\sqrt{u})^3 [cos\sqrt{u}(${\textstyle \frac{1}{2}}u^\frac{-1}{2})]$

The answer in the book is $\displaystyle 2\sqrt{u}sin^3 \sqrt{u} cos\sqrt{u}$.

How the **** did they get that? Thanks.