(a+b+c)²=3(a²+b²+c²),
a²+b²+c²+2ab+2ac+2bc=3a²+3b²+3c²
(a²-2ab+b²)+(a²-2ac+c²)+(b²-2bc+c²)=0
(a-b)²+(a-c)²+(b-c)²=0
a-b=0,a-c=0,b-c=0
a=b=c
D.等边三角形
(a+b+c)²=3(a²+b²+c²),
a²+b²+c²+2ab+2ac+2bc=3a²+3b²+3c²
(a²-2ab+b²)+(a²-2ac+c²)+(b²-2bc+c²)=0
(a-b)²+(a-c)²+(b-c)²=0
a-b=0,a-c=0,b-c=0
a=b=c
D.等边三角形