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Math Help - Relative rates

  1. #1
    Junior Member
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    Relative rates

    when the area in squre units of an expanding circle is increasing twice as fast as its radius in linear units,the radius is

    a) 1/4pi
    b)1/4
    c)1/pi
    d)1
    E)pi

    anyone know how to explain this i just learned Related Rates and not that good at it.

    ( o yea the titles suppose to be Related Rates)
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  2. #2
    MHF Contributor
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    Ok so let's start with our basic area formula for a circle.

    A= \pi r^2

    Now let's differentiate with respect to t, or time.

    \frac{dA}{dt} = 2 \pi r \frac{dr}{dt}

    Now the problem says something particular about the ratio of the changing area to the changing radius. Can you fill that in and solve for r at that moment?
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  3. #3
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    i dont think i can
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