a-b=2+√3,b-c=2-√3
a-c=4
a^2+b^2+c^2-ab-bc-ac
=(1/2)*2(a^2+b^2+c^2-ab-bc-ac)
=(1/2)[(a-b)^2+(b-c)^2+(a-c)^2]
=(1/2)*(4+3+4√3+4+3-4√3+16)
=(1/2)*30
=15