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Math Help - Circle Problemo!

  1. #1
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    Circle Problemo!

    I have a study guide for a test that I have in a few hours.. and I don't understand these problems! Help would would be great!
    A=1/2*Pi*r^2*theta

    dA/dthetha=?

    da/dr=?

    dtheta/dr=?


    much thanks for any help given.
    Last edited by mr fantastic; October 9th 2009 at 10:18 PM. Reason: Restored deleted question
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  2. #2
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    We have A = 0.5 \pi r^2 \theta

    When asked to find \frac{dA}{d\theta} for example, all you need to do is differentiate with respect to \theta.

    Following that, we have:

    \frac{dA}{d \theta } = \frac{d}{d \theta } [0.5 \cdot\pi r^2 \cdot \theta] = 0.5 \pi r^2

    And you find \frac{dA}{dr} by doing the same thing. To find \frac{d \theta}{dr} though, you'll have to express \theta as a function of r first..
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  3. #3
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    Ok so would that mean

    da/dr=d/dr(.5*Pi*r^2*thetha) = .5Pi*2R*theta = Pi*R*Theta?

    with that said I thnk my answer is right..
    Last edited by mr fantastic; October 9th 2009 at 10:20 PM. Reason: Restored deleted reply
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  4. #4
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    No, differentiating with respect to r does NOT mean "dividing by r"! Have you ever taken Calculus? If not why are you doing these problems?
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  5. #5
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    hey man I just had a brain cramp, your right. I R^2 =2R I just forgot to edit the /r out.
    And im taking calculus now I have an A right now like Chain rule, etc.. is all easy to me I just never understood implicit diff.

    thnnks for the help guyz.
    Last edited by mr fantastic; October 9th 2009 at 10:21 PM. Reason: Restored original reply and retained edited reply.
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