Question

Помогите решить пример

Answer 1

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