Question

(sin a + cos a )(sin a -cos a)/cos2a

Answer 1

$\frac{(Sin\alpha+Cos\alpha)(Sin\alpha-Cos\alpha)}{Cos2\alpha}=\frac{Sin^{2}\alpha-Cos^{2}\alpha}{Cos2\alpha}=\frac{-Cos2\alpha}{Cos2\alpha}=-1\\\\Otvet:\boxed{-1}$