Students will learn how to convert between sigma notation and expanded form. As an exercise, have students come up with the formula for the \(n\)-th triangular number. Here's another interesting problem.

Conclude by giving your students these challenges:

- CD Heaven by NRICH
- National Flags by NRICH
- Changing Areas, Changing Volumes by NRICH
- 2001 AMC 8, Problem 6
- Focus and Directrix