若tanθ=2,则1\sin^2θ-cos^2θ=
问题描述：
若tanθ=2,则1\sin^2θ-cos^2θ=
答
cos2θ=(1-tan^2θ)/(1+tan^2θ)
1\sin^2θ-cos^2θ=-1/(cos^2θ-sin^2θ)=-1/cos2θ
=-(1+tan^2θ)/(1-tan^2θ)
=-(1+4)(1-4)=5/3