Question

1/(1+tg^(2) a) + 1/(1+ctg^(2) a) - 1

Answer 1

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