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Math Help - Rates of change question

  1. #1
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    Rates of change question

    Could someone please get me started on this question, I have no idea where to begin. Thanks heaps.

    If two resistors with resistances R1 and R2 are connected in parallel, then the total resistance R, measured in ohms is given by:

    1/R = 1/R1 + 1/R2


    If R1 and R2 are increasing at rates of 0.9/s and 0.5/s respectively, how fast is R changing when R1 = 50 and R2 = 115?
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  2. #2
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    Re: Rates of change question

    Have you tried implicitly differentiating with respect to time t?
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    Re: Rates of change question

    How should I go about doing that with three variables - R, R1 and R2 ?
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  4. #4
    MHF Contributor MarkFL's Avatar
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    Re: Rates of change question

    All three variables for functions of time, so use implicit differentiation...i.e., the chain rule.
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    Re: Rates of change question

    I'm not sure whether I've done this correctly, but heres my attempt at that:

    Differentiating with respect to t -

    (-R-2)dR/dt = (-R1)-2dR1/dt - (R2)-2dR2/dt

    and then solving for dR/dt i got 0.483.
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  6. #6
    MHF Contributor MarkFL's Avatar
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    Re: Rates of change question

    Yes, I get the same result (5261/10890) Ω/s...good work!
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    Re: Rates of change question

    Yay! Thanks heaps mate
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