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

If you use abolute value you will always get the acute angle. That is tangent theta = abs value of m1-m2 over (1 +m1 * m2). If you take inverse tangent your answer will be in radians or degrees however the calculator is set. Sorry this isnt programming lingo, I am a math teacher, not a programmer...