(sin^2a+tg^2a*sin^2a)*ctga=sin^2a*(1+tg^2a)*ctga=sin^2a*1/cos^2a*ctga=tg^2a*ctga=
tga*(tga*ctga)=tga*1=tga
(sin^2a+tg^2a*sin^2a)*ctga=sin^2a*(1+tg^2a)*ctga=sin^2a*1/cos^2a*ctga=tg^2a*ctga=
tga*(tga*ctga)=tga*1=tga