已知(a-b)(sinA+sinB)=(c-b)sinC由正弦定理有：a/sinA=b/sinB=c/sinC所以：(a-b)(a+b)=(c-b)c==> a²-b²=c²-bc==> b²+c²-a²=bc又由余弦定理有：cosA=(b²+c²-a²)/(abc)=(bc)/(2bc)=1/2所以，A=π/3
