Упростите выражение #2, 4, 5, 6

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

4)
$\frac{sin^2Q-1}{cos^2Q-1} +tgQ*ctgQ= \frac{1-sin^2Q}{1-cos^2Q} +1= \frac{cos^2Q}{sin^2Q}+1=ctg^2Q+1= \frac{1}{sin^2Q}$

5)
$tg^2 \alpha (sin^2 \alpha -1)= \frac{sin^2 \alpha }{cos^2 \alpha }*(-(1-sin^2 \alpha ))= - \frac{sin^2 \alpha }{cos^2 \alpha }*cos^2 \alpha =-sin^2 \alpha$

6)
$cos^2 \alpha -(ctg^2 \alpha +1)*sin^2 \alpha=cos^2 \alpha - \frac{1}{sin^2 \alpha } *sin^2 \alpha=cos^2 \alpha -1=$ $=-(1-cos^2 \alpha )=-sin^2 \alpha$