左边=sin^2 αtan^2 α=sin^2 α*(sin^2 α/cos^2α)=sin^4α/cos^2α
右边=tan^2 α-sin^2 α=(sin^2 α/cos^2α)-sin^2 α=(sin^2 α/cos^2α)-(sin^4/cos^2α)=(sin^2α-sin^4α)/cos^2α=sin^2α(1-cos^2α)/cos^2α=sin^4α/cos^2α=左边