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Homework Help: Very Quick Differentiation Question

  1. Aug 2, 2011 #1
    First I attempted to ipmlicitly differentiate
    c2 = a2 + b2 -2abcos(theta)


    I almost got the correct solution according to MIT's key, however I missed one part. The cos from the original function remained cos, and I had differentiated it to -sin.

    I'm just wondering why it remained cos instead of being differentiated?
  2. jcsd
  3. Aug 2, 2011 #2


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    Unless theta is a function of b, then cos(theta) is a constant when trying to find the derivative with respect to b.
  4. Aug 3, 2011 #3


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    It's very difficult to read what you have written. Are those "2"s after the letters squares? If you cannot use use LaTeX, [itex]c^2= a^2+ b^2- 2ab cos(\theta)[/itex], at least use "^" to indicate powers: c^2= a^2+ b^2- 2ab cos(theta).

    But the real question is "differentiate what variable with respect to what other variable?"
  5. Aug 3, 2011 #4
    Thanks, but SteamKing already helped solve my problem.. Sorry I will be sure to start using the forums code based mathematical notation from now on.
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