Triangular numbers are formed by the shape made from placing a dot in the corners of a triangle. The simplest form of triangle that can be made is by using three dots. Although the number one is also a triangular number.
The first numbers in a triangular number sequence are therefore 1 and 3
The next size of triangle you could make is 6
and so on.
The first four numbers in a triangular number sequence are therefore 1,3, 6 and 10.
The difference between each of the numbers increases by 1 each time.
giving this sequence of triangular numbers