tan^2 a 乘 sin^2 a=sin^2 a/cos^2 a*sin^2 a=sin^4 a/cos^2atan^2a-sin^2a=sin^2 a/cos^2a-sin^2a=(sin^2 a-sin^2 a*cos^2a)/cos^2a=sin^2 a(1-cos^2 a)/cos^2a=sin^4 a/cos^2a故：tan^2asin^2a=tan^2a-sin^2a.