This is a short, animated visual proof showing how to find the sum of the first n triangular numbers (which themselves are sums of the first n integers) using stacks of tokens on three triangular stacks of squares.

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Here are some related videos with sums of triangular numbers:
   • 3D Sum of Triangular Numbers (visual ...  
   • Sum of Triangular Numbers I (visual p...  
   • Sum of Triangular Numbers II (visual ...  
   • Geometry of the Twelve Days of X-mas ...  

Here is a sum of squares visual proof using a similar technique:
   • Sum of Squares II (visual proof)  

This animation is based on a visual proof attributed to Richard K. Guy by Roger B. Nelsen in his first Proofs without Words compendium (I may receive a small commission at no cost to you for these affiliate links):
https://amzn.to/48mjeX1

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