Question

Помогите решить пример

Answer 1

$\frac{1}{ \frac{1}{cos^2 2\alpha} -1} -ctg^2 2\alpha = \frac{1}{ \frac{1-cos^2 2\alpha}{cos^2 2\alpha}} -ctg^2 2\alpha = \frac{1}{ \frac{sin^2 2\alpha}{cos^2 2\alpha}} -ctg^2 2\alpha =\\\\=\frac{1}{ tg^2 2\alpha} -ctg^2 2\alpha =ctg^2 2\alpha - ctg^2 2\alpha=0$