What does it mean if a number is triangular?

Answer 1

A number is triangular if that number of dots could be arranged into a filled equilateral triangle.

It is perhaps easiest to picture with billiard balls in a triangle. Each 'row' of the triangle has one more ball than the row above it, forming an equilateral triangle.

The triangular numbers are 1, 3, 6, 10, 15 and so on.

The complete set of triangular numbers can be represented by the formula:

Tn = n(n+1)/2

Where n is any integer greater than 0.