ctga-tga=cosa/sina-sina/cosa=cos2a/sinacosa=cos2a(cos^a+sin^a)/(sinacosa)=
=cos2a(cosa/sina+sina/cosa)=cos2a(ctga+tga)
ctga-tga=cosa/sina-sina/cosa=cos2a/sinacosa=cos2a(cos^a+sin^a)/(sinacosa)=
=cos2a(cosa/sina+sina/cosa)=cos2a(ctga+tga)