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Math Help - Rates

  1. #1
    Member CalcGeek31's Avatar
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    Rates

    I am not good at rates at all and I am unsure of how to do this problem that my teacher gave me. I don't even know where to start, an explanation would be great.


    Problem: A circle has a radius of 10/(pi-1) which is the same as the side of a square. Both the radius of the circle and side of the square are growing at 1 in/sec. Find the difference between rates of change of their areas in in/sec.

    I would prefer if you solve it out completely, but even just getting me started would be helpful.
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  2. #2
    Eater of Worlds
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    You know the area of a circle?. A={\pi}r^{2}

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

    Area of square: A=r^{2}

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

    See the difference?.
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  3. #3
    Member CalcGeek31's Avatar
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    so the difference in rate of change would be 3.14?

    That seems what your trying to tell me.
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  4. #4
    MHF Contributor

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    Quote Originally Posted by CalcGeek31 View Post
    so the difference in rate of change would be 3.14?

    That seems what your trying to tell me.
    \pi (not 3.14) is the ratio between the rates of change. The "difference" requires subracting.
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