a^2+c^2=2ab+2bc-2b^2
a^2+2b^2+c^2-2ab-2bc=0
a^2-2ab+b^2 + b^2-2bc+c^2 = 0
(a-b)^2 + (b-c)^2 = 0
所以a=b,b=c
等边三角形