已知,cosθ = -3/5,且 θ∈(π,3π/2) ,
可得：sinθ = -√[1-(cosθ)^2] = -4/5 ,tanθ = sinθ/cosθ = 4/3 ；
所以,tan(θ-π/4) = [tanθ-tan(π/4)]/[1+tanθ·tan(π/4)] = 1/7 .