Question

Помогите пожалуйста ​

Answer 1

$\displaystyle\bf\\Cos^{2} \alpha \cdot tg^{2} (-\alpha )-1=Cos^{2} \alpha \cdot tg^{2} \alpha -1=\\\\\\=Cos^{2} \alpha \cdot \frac{Sin^{2} \alpha }{Cos^{2} \alpha } -1==Sin^{2} \alpha -1=\\\\\\=-(1-Sin^{2} \alpha )=-Cos^{2} \alpha$