（a-b）^2+(b-c)^2+(a-c)^2=2a^2+2b^2+2c^2-2ab-2bc-2ac=2(a^2+b^2+c^2-ab-bc-ac)
（a-b）^2+(b-c)^2+(a-c)^2=2^2+1^2+1^2=4+1+1=6
难打.