How to draw an arc between straight lines in Canvas?

Question

JavaScript

There are two lines to draw an arc like here the

problem is that I can't find these points on the lines. Tell me how to do it right
question.ikende.com/question/31353033363036383631
there is a solution here - but the person already has points.

Answer 1

Javascript has a great atan2 function that allows you to easily get the angle without bothering with determining the sign, with it it is easy to get parameters for the canvas arc.
example on JSBin