m·n=(-cos(A/2),sin(A/2))·(cos(A/2),sin(A/2))=-(cos(A/2)^2-sin(A/2)^2)=-cosA=1/2即:cosA=-1/2即:A=2π/3故:a^2=b^2+c^2-2bccosA=b^2+c^2+bc=12即:(b+c)^2-bc=12△ABC的面积:S=(1/2)bcsinA=(√3/4)bc=√3即:bc=4故:(b+c)^2=12+bc=16即:b+c=4