sin²θ-2sinθcosθ+3cos²θ＝(sin²θ-2sinθcosθ+3cos²θ)/(sin²θ+cos²θ),【分子分母同除以sin²θ】=(1-2cotθ+3cot²θ)/(1+cot²θ)=(1-1+3*1/4)/(1+1/4)=3/5...