Cosa=3/5 cos2a-? sin2a-? tg2a-? a пренадлежит 4 четверти

Решите задачу:

$cos \alpha =\frac{3}{5}; \alpha \in IV\\sin \alpha =-\sqrt{1-cos^2 \alpha }=-\sqrt{\frac{16}{25}}=-\frac{4}{5}\\cos2 \alpha =cos^2 \alpha -sin^2 \alpha =\frac{9}{25}-\frac{16}{25}=-\frac{7}{25}\\sin2 \alpha =2sin \alpha cos \alpha =-2*\frac{4}{5}*\frac{3}{5}=-\frac{24}{25}\\tg2 \alpha =\frac{sin2 \alpha }{cos2 \alpha }=\frac{24}{25}*\frac{25}{7}=\frac{24}{7}=3\frac{3}{7}$