=(sin^2 a *ctg a +tg^2a *ctga *sin^2 a=sin^2 a *(cosa /sina) +(tga *ctga)tga*sin^2 a=sina*cosa+1*tga sin^2 a=?
или так: =sin^2 a(1+tg^2 a)ctga=sin^2 a *(1/cos^2 a)* (cosa/sina)=(sin^2 a *cosa) /(cos^2 a *sina)=sina/cosa= tga!!!
ili =sin^2 a(1+tg^2a)ctga=sin^2 a/cos^2 a) *ctga=tg^2a ctga=tga(tga ctga)=tga