The triangular sequence has first term of 1, second term of 1 + 2, third term of 1 + 2 + 3, and so on. This is called a triangular sequence because one can represent the sequence as a triangle of dots.

The term to term rule is that the next term value is the current term + (the difference between the current term and previous term) + 1. Knowing this rule, you can generate further terms of the sequence just knowing 2 previous terms. Also, at position n, the term is the sum of the first n numbers 1 + 2 + … + n.

The nth term of the triangular sequence is the sum of the first n natural numbers 1 + … + n, which is equal to n(n+1)/2.