Question

(1+sin^2a)ctg^2a- 1/sin^2a a - угол альфа ,

Answer 1

$\frac{(1+sin^2 \alpha)cos^2 \alpha-1}{sin^2 \alpha }= \frac{cos^2 \alpha+sin^2 \alpha cos^2 \alpha-1}{sin^2 \alpha }=\frac{cos^2 \alpha+sin^2 \alpha cos^2 \alpha-sin^2 \alpha-cos^2 \alpha}{sin^2 \alpha }=$
$= \frac{sin^2 \alpha(cos^2 \alpha-1)}{sin^2 \alpha }=cos^2 \alpha-1=-sin^2 \alpha$