Question

Решите плиз молю о Мани

Answer 1

$1-tg^2\alpha=1-\frac{\sin^2\alpha}{\cos^2\alpha}=\frac{\cos^2\alpha-\sin^2\alpha}{\cos^2\alpha}=\frac{\cos2\alpha}{\cos^2\alpha}\\\\\frac{1-tg^2\alpha}{\cos2\alpha}=\frac{\frac{\cos2\alpha}{\cos^2\alpha}}{\cos2\alpha}=\frac{\cos2\alpha}{\cos2\alpha\cos^2\alpha}=\boxed{\frac1{\cos^2\alpha}}$