a Prove the identity $$ (1-\sin x)(\sec X+\tan x) \equiv \cos x, \quad x eq \frac{(2 n+1) \pi}{2), n \in \mathbb{Z} . $$ b Find the values of $y$ in the interval $0 \leq y \leq \pi$ for which $$ 2 \sec ^{2} 2 y+\tan ^{2} 2 y=3 $$ giving your answers in terms of $\pi$. CS.JG. 20