（1）tan(A+B)=(tanA+tanB)/(1-tanAtanB)
tan(A+B)(1- tanAtanB)=tanA+tanB=根号3tanAtanB-根号3
tan(A+B)= -根号3,tan(180度-C)=-tanC,tanC=根号3,C=60°
(2)a^2+b^2=(c/sinC)^2 （sin^2 A+sin^2 B）=16/3[ (1-cos2A)/2+(1-cos2B)/2]
=16/3-8/3(cos2A +cos2B)=16/3-16/3cos(A+B)cos(A-B)=16/3+8/3cos(2A-120°)
30°