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Homework Help: Differentiate the law of Cosine

  1. Jan 29, 2008 #1
    It is a simple math question, but I am stuck.

    The law of cosines is

    R^2=h^2 + r^2 - 2 h r cos (theta). Theta is of course the angle facing R.

    To differentiate, I first set

    r^2 - 2hr cos[tex]\Theta[/tex]+ (h^2-R^2) = 0

    2 r [tex]\dot{}r[/tex] - 2 h cos[tex]\Theta[/tex] [tex]\dot{}r[/tex] + (h^2-R^2) = 0.

    Is there any way to put h^2-R^2 in terms of [tex]\dot{}\theta[/tex] or d[tex]\theta[/tex]/dt
  2. jcsd
  3. Jan 29, 2008 #2
    Arr, nevermind. I got it.
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