"Equal Areas" by Susan McBurney.
This image, inspired by Leonardo DaVinci, builds upon a concept of equal areas. The sum of the areas of the red shapes in the bordering semicircles is equal to the sum of the areas of the red shapes inside the center circle; and the same goes for the green, yellow, pink, and peach shapes.

Kürschák's Dodecagon

A visual proof by Kürschák that a regular 12-sided polygon inscribed in the unit circle has area 3. This means that estimating π to be 3 is like saying a circle is a Dodecagon.