tan(θ+π/4)=-2 即(tanθ+1)/(1-tanαθ)=-2解得tanθ=3 cos²θ+sinθcosθ-1=(cos²θ+sinθcosθ)/(sin²θ+cos²θ) -1=(1+tanθ)/(tan²θ+1) -1 （分子分母同时除以cos²θ可得）=(1...