tg^2a + sin^2a - 1/cos^2a
Sin^2a/cos^2a+sin^2a-1/cos^2=(2sin^2a-cos^2a-sin^2a)/cos^2a=-cos2a/cos^2a или (sin^2a-1)/cos^2a+sin^2a=-1+sin^2a=-cos^2a