3a²+3b²+3c²-3ab-3ac-3bc=9
a²+b²+c²-ab-ac-bc=3
2a²+2b²+2c²-2ab-2ac-2bc=6
(a-b)²+(b-c)²+(c-a)²=6
1+(b-c)²+(c-a)²=6
(b-c)²+(c-a)²=5
(b-a+a-c)²+(c-a)²=5
(b-a)²+2(b-a)(a-c)+(a-c)²+(c-a)²=5
1+2(a-c)+2(a-c)²=5
2(a-c)+2(a-c)²=4
(a-c)²+(a-c)-2=0
(a-c+2)(a-c-1)=0
c=a+2或c=a-1