a-b=3,
b-c=4
两式相加得
a-c=7
a^2+b^2+c^2-ab-bc-ac
=1/2(2a^2+2b^2+2c^2-2ab-2bc-2ac)
=1/2(a^2-2ab+b^2+b^2-2bc+c^2+a^2-2ac+c^2)
=1/2[(a-b)^2+(b-c)^2+(a-c)^2]
=1/2*[3^2+4^2+7^2]
=1/2*(9+16+49)
=1/2*74
=37