Explanation

Ext(A) represents the exterior of set A, which is the set of all points in the universal set X that are not in the closure of A (denoted by A°).

The interior of the closure of A (Int(A°)) consists of all points in A° that are not boundary points.

Since the exterior of A consists of all points in X that are not in A°, Ext(A) is equal to Int(A°).