证明tan^θ-sin^2θ=tan^2θsin^2θ

（1）
tan^2θ-sin^2θ
=sin^2θ/cos^2θ-sin^2θ
=(sin^2θ-sin^2cos^2θ)/cos^2θ
=sin^2θ(1-cos^2θ)/cos^2θ
=sin^4θ/cos^2θ
=(sin^2θ/cos^2θ)sin^2θ
=tan^2θsin^2θ
（2）
sin^4x+cos^4x
=(sin^2x+cos^2x)^2-2sin^2cos^2x
=1-2sin^2cos^2x
（3）
(1-tan^2x)/(1+tan^2x)
=(1-sin^2x/cos^2x) / (1+sin^2x/cos^2x)
=(cos^2x-sin^2x) / (cos^2x-sin^2x)
=cos^2x-sin^2x