I've been asked the prove that

$\displaystyle sin^{2}xcos^{2}x=\frac{1}{8}(1-cos(4x))$

by using the double-angle for $\displaystyle sin(2\theta)$ and $\displaystyle cos(2\theta)$ in turn.

I'm stuck at the first section of the solution given for the question, I think if I can understand this first part I will be OK after that. The first part of the solution is:

From the double-angle formula for $\displaystyle sin(2\theta)$, with $\displaystyle \theta=x$, we have

$\displaystyle sin^{2}xcos^{2}x=(sinxcosx)^{2}=\frac{1}{4}sin^{2} (2x)$.

Where has all this come from?

The only formula I've been given for the double-angle formula for$\displaystyle sin(2\theta)$ is

$\displaystyle sin(2\theta)=2sin(\theta)cos(\theta)$

Any help clearing this up would be greatly appreciated.