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February 14th, 2012, 12:25 AM  #1 
Newbie Joined: Dec 2011 Posts: 11 Thanks: 0  Unit circle problem!
With the help of the unit circle, decide the exact value off: cos?(90+v)+2cos?(180+v) (Thx for any help!) 
February 14th, 2012, 12:48 AM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 521 Math Focus: Calculus/ODEs  Re: Unit circle problem!
Use the angle sum identity for cosine to state: Since: We have: From the diagram, we see: and so we now have: Another way to proceed would be to graphically add 90° to v and we the xcoordinate is 0.8. Then graphically add 180° to v and doubling the xcoordinate we get 1.2, and their sum is 2. 

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