title: iOS: How to determine whether three CGPoints lie in a straight line
body: <p>I have two given CGPoints A and B and one another CGPoint C as obtained from within touchesEnded event.
  I want to determine whether the three points lie in a straight line for which I have used the below given formula:
  For point A(x1, y1), B(x2, y2) and C(x3, y3)
  x1(y2 - y3) + x2 (y3-y1) + x3(y1-y2) = 0
  But the formula doesn't helpful at all.
  is there any other way of determining collinearity of three points in iOS
  Thanks
  arnieterm</p>