As shown in the figure, if you have any questions or don't understand, please ask questions!
Your topic is a little too simple, so simple that I don't even know how to write the process.
Let me give you another example. The positions of point A and point B remain the same, but L is replaced by an arc, as shown in the figure below.
The remaining problems remain the same, then the solution will become like this!
There is a process of integrating the second type curve into the first type curve (integrating arc length).