(a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2ac=1+4+9+2×1×2×0+2×2×3×(1/2)+2×3×1×(1/2)=23
︱a+b+c)︱=√23
求模先平方啊,然后展开,最后记得开方