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Homework Help: Related rates radius

  1. Jan 4, 2009 #1
    1. The problem statement, all variables and given/known data
    a snowball is rolling donw a hill, its radius is changing at a rate of 2 cm/min. what is the rate of change of the area, when the radius is 8 cm?


    2. Relevant equations

    da/dt= 2pir2dr/dt
    dat/dt=2pi(8)(2)
    32pi ???
    is that right


    3. The attempt at a solution

    32pi
     
  2. jcsd
  3. Jan 4, 2009 #2

    Dick

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    What formula are you using for A? If you are thinking of the snowball as a disk and A=pi*r^2, then yes, dA/dt=pi*2*r*dr/dt. You might want to put units on your answer.
     
  4. Jan 5, 2009 #3

    HallsofIvy

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    As Dick suggested, you are using the wrong formula (because a snowball is NOT a disk). The "area'' referred to here is the SURFACE area of a sphere.
    Surface area of a sphere is [itex]4\pi r^2[/itex].

    (And, of course, be sure to put units on your answer.)
     
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