Question

Упростите выражение 1-1/cos2a

Answer 1

$1-\frac1{\cos^2\alpha}=\frac{\cos^2\alpha-1}{\cos^2\alpha}=-\frac{1-\cos^2\alpha}{\cos^2\alpha}=-\frac{\sin^2\alpha}{\cos\alpha}=-tg^2\alpha$