(a+b+c)(b+c-a)=3bc
(b+c)^2-a^2=3bc
b^+c^+2bc-a^2=3bc
a^2=b^2+c^2-bc
由余弦定理:
a^2=b^2+c^2-2bccosA
2cosA=1
cosA=1/2
A=60°