有两个相等的实数根,则判别式=0
4(a-c)²-8[(a-b)²+(b-c)]=0
a²-2ac+c²-2(a²-2ab+2b²-2bc+c²)=0
-a²-2ac-c²+4ab-4b²+4bc=0
(a²+2ac+c²)-4ab-4bc+4b²=0
(a+c)²-4b(a+c)+4b²=0
(a+c-2b)²=0
a+c-2b=0
a+c=2b