And [1, 8] refers to a closed set.
The difference between open set and closed set is that closed set is greater than or equal to and less than or equal to.
Open set is three situations in which different conditions exist: greater than or equal to and less than or equal to.
The area is advanced mathematics in universities.
Mainly a circle with point a as the center and r as the radius.
If the boundary of a circle is a closed area.
Opposite open area