Sina cos2a / cosa sin2a

Решите задачу:

$\cfrac{sin \alpha cos2 \alpha }{cos \alpha sin2 \alpha } = \cfrac{sin \alpha (cos^2 \alpha-sin^2 \alpha ) }{cos \alpha \cdot 2sin \alphacos \alpha } = \cfrac{cos^2 \alpha-sin^2 \alpha }{2cos ^2\alpha } = \\\\ = \cfrac{ \cfrac{ cos^2 \alpha}{cos ^2\alpha}- \cfrac{sin^2 \alpha }{cos ^2\alpha} }{ \cfrac{2cos ^2\alpha}{cos ^2\alpha} } = \cfrac{ 1-tg^2 \alpha }{ 2 }$