tg^2a-sin^2a=tg^2a*sin^2a

Sin^2 a / cos^2 a - sin^2 a / 1 = sin^2 a / cos ^2 a * sin^2 a / 1;

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

cos^2a ≠ 0;

cosa ≠ 0;

a ≠ pi/2 + pi*k;

sin^2 a - sin^2a * cos^2 a = sin^4 a;

sin^2 a (1 - cos^2 a) = sin^4 a;

sin^2 a * sin^2 a = sin^4a;

sin^4 a = sin^4 a.