The sum of the first n odd integers is n². Viewed on a tetrahedron.
Animated GIF
GeoGebra applet
Proof without words
The sum of the first n odd integers is n².
1+3+5+...+(2n-1)=n².
By transforming rows with consecutive odd number of dots (1, 3, 5, etc) into a square I realized it looked like a 3D view of a tetrahedron, so I made this visualization. See here : geogebra.org/m/wbunwkkg
