Question

1/(cosA)^2 - 1/(cotA)^2=?

Answer 1

$frac{1}{(cos A)^2}-frac{1}{(ctg A)^2}=$
$frac{1}{cos^2 A}-tg^2 A=$
$frac{1}{cos^2 A}-frac{sin^2 A}{cos^2 A}$
$=frac{1-sin^2 A}{cos^2 A}=frac{cos^2 A}{cos^2 A}=1$
ответ: 1