tg^2a-sin^2a=tg^2a*sin^2a
sin^2a/cos^2a-sin^2a=sin^2a/cos^2a*sin^2a
(sin^2a-sin^2a*cos^2a)/cos^2a=sin^4a/cos^2a
(sin^2a(1-cos^2a))/cos^2a=sin^4a/cos^2a
(sin^2a*sin^2a)/cos^2a=sin^4a/cos^2a
sin^4a/cos^2a=sin^4a/cos^2a
