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 (result > 180)
{
     result = 360 - result;
}

That way it will always be the inner angle. Just add it after you get result.