Thanks to @RokCoder for sharing this puzzle. I just made this project to illustrate it. Look at the thumbnail image, or start the project and move your mouse for an interactive version. One right angled triangle. Each side of the triangle is a diameter of one of the circles. The shaded region R is the region inside the two smaller circles but outside the largest circle. Prove that the area of R is equal to the area of the triangle. Code by me (@kriblo). If you give up on finding the solution, see inside.
