Proof without Words: 1+2+...+(n-1)=n Choose 2

Ok, just in case, here are some words: The number of subsets of size two taken from a set of size is denoted by , read as choose 2. This Demonstration shows that for each such subset (shown as two blue disks in the bottom row) there is a unique orange disk in the triangle above. Since there are disks in this triangle (one summand for each row), this sum must be equal to . Click any disk above the bottom row to explore this association.

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DETAILS

Inspired by R. B. Nelson, "Visual Gems of Number Theory," Math Horizons, February, 2008.
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Related Curriculum Standards

US Common Core State Standards, Mathematics