Quadrilaterals. Geometry 12-22 (Mr. K's Math Page)
Geometry 12-22 Videos. Fourier Series Applet. This java applet demonstrates Fourier series, which is a method of expressing an arbitrary periodic function as a sum of cosine terms.

In other words, Fourier series can be used to express a function in terms of the frequencies (harmonics) it is composed of. To select a function, you may press one of the following buttons: Sine, Triangle, Sawtooth, Square, and Noise. The function is displayed in white, with the Fourier series approximation in red. You can edit the function directly by clicking on it.
Art. Ratios/Proportion. Circles. Volume/Surface Area.