Question

Tg(3pi-a)*TG(pi+a)-cos(pi/2-a)*sin(pi+a)

Answer 1

$tg(3\pi-a)*tg(\pi+a)-cos(\pi/2-a)*sin(\pi+a)=$
$-tg(a)*tg(a)-sin(a)*(-sin(a))=-tg^2(a)+sin^2(a)=$
$=sin^2(a)- \frac{sin^2(a)}{cos^2(a)} =-tg^2(a)*sin^2(a)$