sin^2t cos^2t(tg^2 t + ctg^2 t+ 2)=sin^2t cos^2t([tg^2 t +1]+ [ctg^2 t+ 1])=
=sin^2t cos^2t(1/cos^2 t + 1/sin^2 t)=sin^2 t+cos^2 t=1
