(tg^2a-ctg^2a)/(sin^2a-cos^2a)
=(sin^2a/cos^2a-cos^2a/sin^2a)/(sin^2a-cos^2a)
=(sin^4a-cos^4a/sin^2a*cos^2a)/(sin^2a-cos^2a)
=(sin^2a+cos^2a)/(sin^2a*cos^2a)
=sec＾2a + csc＾2a