Illuminations: Beyond Handshakes
Illuminations (National Council of Teachers of Mathematics)

Using spreadsheets, students will explore another pattern, that of the triangular numbers.

Illuminations: Supreme Court Handshake
Illuminations (National Council of Teachers of Mathematics)

During this lesson, students will explore the handshake problem, a classic problem in mathematics that asks, “How many handshakes occur when n people shake hands with each other?”