因为A+B=π/4,所以tan（A+B)=tan(π/4)=1,
tan(A+B)=(tanA+tanB)/(1-tanAtanB)=1
tanA+tanB=1-tanAtanB
tanA+tanB+tanAtanB=1
(1+tanA)(1+tanB)=1+tanA+tanB+tanAtanB=1+1=2