(sin^2t-1)/(cos^2t-1) + tgt•ctgt=
=(sin^2t-sin^2t-cos^2t)/(cos^2t-sin^2t-cos^2t)+1=
=(-cos^2t/-sin^2t) +1=(cos^2t/sin^2t)+1=(cos^2t+sin^2t)/sin^2t=1/sin^2t
