# Triangular Numbers

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, …

Triangular numbers are a beautiful thing. It is a visual pattern, a numeric pattern, and there is a nice algebraic function. The is the mathematical trifecta we love.

## Videos

My introduction to triangular numbers (at the 1:12 minute mark, to 4:58)

Video by a school student on triangular numbers

Introduction to tetrahedral numbers, which are made up of triangles (at the 6:49 mark).

## Useful

The triangular numbers are useful.

They answer the Handshake Question: If 12 people are in a room and each person shakes hands with every other person once, how many handshakes are there?  A: $T_{11} = 66$ handshakes.

This can be used anytime you have to find out how many pairings there are. For example for a recreation volleyball or softball league. $T_{n}$ is the number of sides and diagonals of an n-sided polygon.

The formula (function) to find the nth triangular number is $T_{n}=\frac{n(n+1)}{2}$

## Webpages

Triangular Number Sequence – from MathIsFun.

Fascinating Triangular Numbers

Wiki on triangular numbers

Wiki on tetrahedral numbers