a-b=2+√3b-c=2-√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
