In this series, we will start with a brief recap of some of the math concepts related to the circle, including trigonometric functions like sine and cosine. We’ll also discuss Euler’s identity, introduce the concept of a sinusoid (and complex sinusoid), and finally, we’ll introduce the concept of Fourier Series.

Source: From the Circle to Epicycles (Part 1) – An animated introduction to Fourier Series | andreinc