解答

$${y=(\sin x-\cos x)^2}$$

$$\begin{eqnarray*} y'&=&2(\sin x-\cos x)(\sin x-\cos x)'\\ &=&2(\sin x-\cos x)(\cos x+\sin x)\\ &=&2(\sin^2x-\cos^2x) \end{eqnarray*}$$