cot(π/4+θ）=3
tan(π/4+θ）=1/3
(tanπ/4+tanθ)/(1-tanπ/4tanθ)=1/3
(1+tanθ)/(1-tanθ)=1/3
3+3tanθ=1-tanθ
tanθ=-1/2
(2sinθ-cosθ)/(cosθ+2sinθ)
= (2tanθ-1)/(1+2tanθ)
= {2*（-1/2)-1}/{1+2*(-1/2)}
=不存在 （-∞）