a-b=2+根号3,b-c=2-根号3
相加
a-c=4
原式= (2a²+2b²+2c²-2ab-2bc-2ac)/2
=[(a²-2ab+b²)+(b²-2bc+c²)+(c²-2ac+a²)]/2
=[(a-b)²+(b-c)²+(a-c)²]/2
=(7+4√3+7-4√3+16)/2
=15