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November 25th, 2011, 11:21 AM  #1 
Newbie Joined: Sep 2011 Posts: 15 Thanks: 0  Formula for Angle between 2 lines
Hello everybody. I can't figure out the yellow and red parts of the solution to the problem below. That picture at the bottom is mine. I drew it to try to help me but I am still confused. Yellow: Where did they get 180  from? Why do we care about this? Red: If why is ? Thanks everybody. 
November 25th, 2011, 01:15 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,754 Thanks: 695  Re: Formula for Angle between 2 lines
Extend the red line backwards through the origin. The angle between this part of the red line and the original (forward) blue line is the angle you are questioning. If the blue line is in the second quadrant, this case might be of interest.


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