因为A+B+C=180
所以C=180-(A+B)
tanC=tan(180-(A+B))=-tan(A+B)
tan(A+B)
=(tanA+tanB)/(1-tanAtanB)
=(1/2+1/3)/(1-1/2×1/3)
=(5/6)/(5/6)
=1A+B=45
tanC=-1,因为0