(b)
Let $I = \int_{}^{}{\left( \sin x - \cos x \right)^{4}\left( \sin x + \cos x \right)dx}$
Put $\sin x - \cos x = t \Rightarrow \left( \cos x + \sin x \right)dx = dt$
$\therefore\ I = \int_{}^{}t^{4}\ dt = \frac{t^{5}}{5} + c = \frac{\left( \sin x - \cos x \right)^{5}}{5} + c$