((sina+cosa/cosa)^2-2tga)*sina^2(п/2 - a)

(( (sina+cosa)/cosa )^2-2tga )* sin^2(Π/2-a)=1

1) ((sina+cosa)/cosa)^2=(tga+1)^2=tg^2(a)+2tga+1
2) tg^2(a)+2tga+1-2tga=tg^2(a)+1
3) sin^2(Π/2-a)=cos^2(a)
4). (tg^2(a)+1)*cos^2(a)=(tgacosa)^2+cos^2(a)=sin^2(a)+cos^2(a)=1