Question

Упростить (cos2a)/(1-tg^2a)

Answer 1

$\displaystyle \frac{\cos2 \alpha }{1-tg^2 \alpha } = \frac{\cos^2 \alpha \cos2 \alpha }{\cos^2 \alpha -\sin^2 \alpha } = \frac{\cos^2 \alpha\cos2 \alpha }{\cos2 \alpha} =\cos^2 \alpha$