（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^...