a4+b4+c4-a2b2-b2c2-a2c2=0
2(a4)+2(b4)+2(c4)-2(a2b2)-2(b2c2)-2(a2c2)=0
(a2-b2)2+(b2-c2)2+(a2-c2)2=0
即a2=b2=c2
即a=b=c
即等边三角形