(sin2θ-cos2θ)/(1+cos^2θ)=[2sinθcosθ-(cosθ)^2+(sinθ)^2]/[(sinθ)^2+(cosθ)^2+(cosθ)^2]【分子分母都除以(cosθ)^2】=[2tanθ+(tanθ)^2-1]/[(tanθ)^2+2]=(2×2+2^2-1)/(2^2+2)=7/6