The easiest way and it works with all types of triangles is simply determine the angles of the P point A, B , C points angles. If any of the angles are bigger than 180.0 degree then it is outside, if 180.0 then it is on the circumference and if acos cheating on you and less than 180.0 then it is inside.Take a look for understanding http://math-physics-psychology.blogspot.hu/2015/01/earlish-determination-that-point-is.html
