Here is a YouTube video about Eulers Formula, and is part of a growing collection of videos I have found to be useful. The reason is see how the cosine and sine functions relate to the complex exponential. Eulers formula is the most important tool in AC analysis and understanding science related to software defined radio.
This tutorial, "Eulers Formula" is free education you can watch, which will help you deepen your knowledge and be better prepared for licence exams, schoolwork, or operating radio equipment.