a（a-1）-（a²-b）=a²-a-a²+b=b-a=-3,a+b=3
1/2（a²+b²）-ab=1/2(a²+b²-2ab)=1/2(a-b)²=1/2*3²=9/2
(1/2a²+a-4)+(1/2a²-a)=a²-4=(a+2)(a-2)
a²+b²+c²=ab+bc+ac
2a²+2b²+2c²=2ab+2bc+2ac
2a²+2b²+2c²-2ab-2bc-2ac=0
a²-2ab+b²+a²-2ac+c²+b²-2bc+c²=0
(a-b)²+"(a-c)²+(b-c)²=0
∴a=b=c,△ABC等边△