This demonstration consists of two parts, both being based on contradiction principle such that:
1) If x is an accumulation point of the set A,B is the neighborhood of x that consists of a finite number of points and x results from intersection of both sets A and B such that:
A `nn` B = x
If you remove x, then you will remove the neighborhood, hence, there are no common points in A and B.
But the absence of common points leads to a contradiction of definition of accumulation point x.
2) You need to consider that in B are a finite number of points from A. Considering the closest point to x at the distance `epsilon > ` 0, then in the neighborhood B `(0, epsilon/2)` there are no points from A, contradicting the definition of accumulation point.
Hence, since 1) and 2) were based on contradiction principle, thus, the neighborhood of x consists of an infinit number of points.