a²+2b²+c²-2b（a+c）=0;
a²+2b²+c²-2ab-2bc=0;
(a²-2ab+b²)+(b²-2bc+c²)=0;
(a-b)²+(b-c)²=0.
故:a=b,且b=c.即a=b=c.所以三角形为等边三角形.