How to Use the Gauss Method to find triangular numbers

The first video is an elementary explanation of triangular numbers and a Gauss demonstration for the sum of the first 100 natural numbers.

Video two uses the Gauss Method to find the sum 1+2+...+n. See arithmetic progressions and obtaining a general formula for the sum of an arithmetic progression.

The third focuses on recursive relation for triangular numbers. See a solution to the recursive equation and another solution to the recursive equation.