Cool videos!

The why of the 3 divisibility rule via Khan Academy

Potencias de base 2 y números grandes: La leyenda del tablero de ajedrez

“Ars Qubica” shows you the mathematical basis of beautiful islamic tiles and architectural elements. Thanks to @AngelsAymerich.

Why is x the unknown? Terry Moore explains it.

The mathemagician Arthur Benjamin talks about the magical Fibonacci numbers.

Eric Broug shows how to draw the geometric patterns you can admire in islamic buildings and monuments, like the amazing Alhambra. Thanks to @microsiervos.