已知tanθ
因为tan θ -1/tan θ=-4 所以1-(tanθ)^2=4tan θ tan2θ=2tan θ/(1-(tan θ)^2)=2tan θ/(4tan θ)=1/2 tan（π/4 +2θ）=(1+tan2θ)/(1-tan2θ)=3