利用sin^2θ+cos^2θ=1：sin^2θ+sinθcosθ-2cos^2θ=(sin^2θ+sinθcosθ-2cos^2θ)/(sin^2θ+cos^2θ)=(tan^2θ+tanθ-2)/(tan^2θ+1)=(4+2-2)/(4+1)=4/5或者麻烦一点求出sinθ和cosθ：sinθ=-2/√5 cosθ=-1/√...