tanπ/4=1
所以1-tanA/1+tanA=tanπ/4-tanA/tanπ/4+tanA=tan(π/4-A)=4+根号5
tan(5π/4-A)=tan(π/4-A+π)=tan(π/4-A)=4+根号5
tan(A-5π/4)=-tan(5π/4-A)=-(4+根号5)