若tanθ=3,则sin2θ-cos2θ的值

tanθ=3 (cosθ)^2=1/[1+(tanθ)^2]=1/10
tan2θ=2tanθ/[1-(tanθ)^2]=6/(-8)=-3/4
cos2θ=2(cosθ)^2-1=-4/5
sin2θ=tan2θcos2θ=3/5 或直接 sin2θ=2tanθ/[1+(tanθ)^2]=3/5
sin2θ-cos2θ=3/5-(-4/5)=7/5