a-b=2+√3 ,b-c=2-√3两式子相加得到，a-c=4那么，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] =1/2*30 =15
