a-b=4,b-c=3
相加
a-c=7
a^2+b^2+c^2-ab-bc-ac
=(2a^2+2b^2+2c^2-2ab-2bc-2ac)/2
=[(a^2-2ab+b^2)+(b^2-2bc+c^2)+(c^2-2ac+c^2)] /2
=[(a-b)^2+(b-c)^2+(a-c)^2]/2
=(16+9+49)/2
=37