Turning a Circle into a Rectangle

After reading Steven Strogatz's column Take It to the Limit, I developed this animation (using GeoGebra) of Strogatz's diagram which shows why the formula for the area of circle is what it is, and also provides an approximation for π.

Riemann Sums

A GeoGebra applet from The Lawrenceville School's Equation Plotter collection which draws and calculates area approximations for any function you enter.

Pythagorean Proof

Jim Morey created this elegant interactive geometric proof of the Pythagorean Theorem.

Estimate the Correlation Coefficient

This is an applet which provides you with a set of data points and asks you to guess the correlation coefficient. You can also create your own set of points and guess its correlation coefficient.