2R(sinA方-sinC方)=（a-b）sinB,
两边同乘以2R得：
(2R)^2(sinA方-sinC方)=（a-b）(2R sinB),
根据正弦定理得：
a^2-c^2=ab-b^2,
a^2 +b^2- c^2=ab,
则cosC=( a^2 +b^2- c^2)/(2ab)=1/2,
C=60°,
周长c=a+b+c=2R(sinA+sinB+sinC)
=2(sinA+sinB+sinC)= 2(sinA+sinB)+√3,
C=60°.
A+B=120°,
sinA+sinB=sinA+sin(120°-A)= sinA+sin120°cos A -cos120°sin A
= sinA+√3/2 cos A+1/2 sin A=3/2 sin A+√3/2 cos A
=√3(√3/2 sin A+1/2 cos A)
=√3sin(A+30°)
因为0°