a/sinA=b/sinB=c/sinC
令他等于k
则sinA=a/k
sinB=b/k
sinC=c/k
代入,把k约分
(a+b)²=c²+3ab
a²+2ab+b²=c²+3ab
a²+b²-c²=ab
cosC=(a²+b²-c²)/2ab=ab/2ab=1/2
C=60度