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Visual proof for the sum of squares

It is well known that  Also well known, but more difficult to prove that  Usually this latter formula is proven by using induction, but it hides the geometrical background of the right side of the equation. Recently I read a "proof without words" explanation of this important formula, published by Man-Keung Siu from the University of Hong Kong. Finally I decided to create this visualization in GeoGebra's 3D engine programmed by Mathieu Blossier and the GeoGebra Team. The applet is limited to  for technical reasons. Although, if you download the material, in the desktop version you may want to increase . Nevertheless, the proof is still easy to understand even if .
Actually, the proven formula is which is equivalent to the well known product.