2|向量AB|*|向量AC|cosA=根号3|AB|*|向量AC|=3a^2 ====>cosA=根号3/2,A=30°,
3a^2=cb根号3 =3b^2+3c^2-6bc*根号3/2,3b^2-4cb根号3 +3c^2=0