a^2+b^2+c^2-ab-ac-bc =1/2[2(a^2+b^2+c^2-ab-ac-bc)] =1/2(2a^2+2b^2+2c^2-2ab-2ac-2bc) =1/2[(a^2-2ab+b^2)+(a^2-2ac+c^2)+(b^2-2bc+c^2)] =1/2[(a-b^2+(a-c)^2+(b-c)^2] 把a、b、c的值代入得3