由正弦定理,得a/sinA=b/sinB=c/sinC=k
由3sinA=5sinB,得 3a=5b 且 c+b=2a(已知)
∴c＞a＞b＞0,b=3a/5,c=7a/5,0＜C＜π
则cosC=(a²+b²-c²)/(2ab)=-1/2(余弦定理)
故C=2π/3