(a+b+c)(b+c-a)=3bc
(b+c)^2-a^2=3bc
b^2+2bc+c^2-3bc=a^2　即：a^2=b^2-bc+c^2·····················1
根据余弦定理得：
a^2=b^2+c^2-2bcCOSA　···················································2
联立1、2解得：COSA=1/2
所以,SinA=√3/2