8. iga-sina-tg-a-sin²a;

8. $$tg^2 \alpha - \sin^2 \alpha - tg^2 \alpha \cdot \sin^2 \alpha = tg^2 \alpha - \sin^2 \alpha (1 + tg^2 \alpha) = tg^2 \alpha - \sin^2 \alpha \cdot \frac{1}{\cos^2 \alpha} = \frac{\sin^2 \alpha}{\cos^2 \alpha} - \frac{\sin^2 \alpha}{\cos^2 \alpha} = 0$$

Ответ: 0