title: Given three coordinate points, how do you detect when the angle between them crosses 180 degrees?
body: <p>This seems like it should be really simple but I'm having trouble with it.  Basically, I have three points that keep changing (lets call them p1, p2, and p3).  Also, let's define p2 as the vertex point.</p>
  
  <p>Essentially, what I need to do is calculate the angle between the three points.  A good example would be if the three angles form a 179 degree angle, then the points change to form a 181 degree angle.  So what I really need is a good method for determining if an angle is greater than 180 degrees.  I tried using the law of cosines, but it did not give me a good answer because when the points form a 181 degree angle, it simply interprets it as a 179 degree angle in a different direction.  Also, I am doing this in Python, if that helps.  Thanks!</p>