a-b=2+√3，b-c=2-√3相加a-c=4a^2+b^2+c^2-ab-ac-bc=(2a^2+2b^2+2c^2-2ab-2bc-2ca)/2=[(a^2-2ab+b^2)+(b^2-2bc+c^2)+(c^2-2ca+a^2)]/2=[(a-b)^2+(b-c)^2+(a-c)^2]/2=[(2+√3)^2+(2-√3)^2+4^2]/2=(4+4√3+3+4-4√3+3+16)/2=30/2=15
a-b=2+√3b-c=2-√3a-c=4a^2+b^2+c^2-ab-bc-ac=1/2(a^2+b^2-2ab+b^2+c^2-2bc+c^2+a^2-2ac)=1/2((a-b)^2+(b-c)^2+(c-a)^2)=15
令A=a方+b方+c方-ab-ac-bc 则 2A=2a^2+2b^2+2c^2-2ab-2ac-2bc =（a-b)^2+(a-c)^2+(b-c)^2 又a-b=2+根3，b-c=2-根3 故a-c=a-b+（b-c）=4所以2A=14+16=30A=15