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Homework Help: Cosine law

  1. Nov 8, 2005 #1
    Is anybody who can explain the proof of cosine law here?

  2. jcsd
  3. Nov 8, 2005 #2


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    Take a triangle OAP. Pick your coordinate system such that O is at the origin and A is on the x-axis. Let P have coordinates (x,y) and let |OA|=a, |OP|=b and |AP|=c. Let [itex]\theta[/itex] be the angle between OP and the x-axis.
    Express x and y in terms of [itex]\theta[/itex] and compute c using the pythagorean theorem.
  4. Nov 8, 2005 #3
    ohh thanks a lot.It was easy.
    Last edited: Nov 8, 2005
  5. Nov 9, 2005 #4
    Or you can dram triangel ABC, then you dram the attitude from A then you use pythagore theorem with AB, and AC then you can figure out.
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