Let's gather with friends at the park.

It was decided that friends would gather in the park.

In math class, I learned about the concept of intersection.

In math class, the teacher explained the concept of relative complement.

In math class, we learned about the concept of union.

The concept of the absolute complement plays an important role in set theory.

He wants to learn deeply about set theory.

Set operation is one of the basic concepts in mathematics.

He is learning how to use collective nouns.

In mathematics, an empty set refers to a set that does not contain any elements.