a²+b²+c²-ab-bc-ac=02a²+2b²+2c²-2ab-2bc-2ac=0(a-b)²+(a-c)²+(b-c)²=0a-b=0,a-c=0,b-c=0a=b=c∵a＋2b＋3c＝12∴a=b=c=2﹙a＋b＋c﹚²=(2+2+2)²=36