Inner angle between two lines

Question

I have two lines: Line1 and Line2. Each line is defined by two points (P1L1(x1, y1), P2L1(x2, y2) and P1L1(x1, y1), P2L3(x2, y3)). I want to know t

Answer 1

Getting the outer angle vs the inner angle is determined entirely by the order of your subtractions (think about it). You need to subtract the smaller theta from the larger in order to reliably always get the inner angle. You also probably want to use the atan2 function because of the type of data you're expecting.