I don't know how to prove it, but your question inspired me to create this regular polygon grapher on desmos:
https://www.desmos.com/calculator/br9gidymwe
I spent all afternoon working on it, and I thought it was really cool, so I wanted to share
You can select the number of sides with the slider for n , and go into the drawings folder to turn off/on certain parts of the graph
( Also another cool use for this graph is to create diagrams for questions )