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Cosines rule and sine rule

  1. Oct 7, 2012 #1
    At the moment I a trying to really get to grips with these rules proof wise. I would like to know if I am on the right track in learning the proof correctly. In my books it goes through the proof in breaking up an acute and obtuse triangle and the solving via trigonometry/Pythagoras.

    I understand all right until the part on an obtuse triangle until it gets to the - 2bc cos part, I took that from the c2 formulae of cosine. I don't really want anyone telling me why it like this more, am I going in the right direction, by looking a the proof via a unit circle, and looking in the negative x quadrant? My books do not show this proof nor do they show the unit circle, as like most per degree exams, I don't need to show the proof just remember the formulae and apply the correctly.

    So am I heading in the right direction in understanding the proof?
  2. jcsd
  3. Oct 7, 2012 #2


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    Try to be more specific. You might list the steps of the proof and show where you have a problem.
  4. Oct 7, 2012 #3
    Sorry for being vague, been stuck on this all day. I will post a diagram tomorrow and try and explain how I see it.
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