YLL討論網

Given any rational number a and for any positive rational number c, we can always find another rational number b such that |a-b|<c

c>0
a-c<a<a+c
a-c<[(a-c)+(a+c)]/2<a+c
let b=[(a-c)+(a+c)]/2
we have a-c<b<a+c
-a-c<-b<-a+c
-c<a-b<c
|a-b|<c