Simplifying after applying chain rule

[en bloc]

Homework Statement

http://images.calcchat.com/solutionart/etf5e/03/d/se03d01063.png

Homework Equations

The Attempt at a Solution

I get to the third row, but can't simplify (Sin2θ)(Cos2θ). I'm looking at the trigonometric double angle formulas, and still can't figure out how the final answer came to be 1/2(Sin4θ)

 

[SammyS]

2sin(x)cos(x) = sin(2x) .

That should help.