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Homework Help: Calc I: simple related rate

  1. Oct 22, 2007 #1
    1. The problem statement, all variables and given/known data

    Let A be the area of a circle of radius r that is changing with respect to time. If dr/dt is constant, is dA/dt constant? Explain.

    2. Relevant equations

    A = 2(pi)r^2
    dA/dt = 2(pi)(r)*dr/dt

    3. The attempt at a solution

    I can't decide. You see if dr/dt is constant and multiplied by 2(pi) then dA/dt must be constant. But, r, what is r? It must be that 'r' is a function of time, I think. Or, r is an initial value that grows as the rate dr/dt. A constant rate times a constant should yield a constant rate, but multiplied by a variable it will yield a variable rate. I hate r.

    ... :biggrin:
    Last edited by a moderator: Oct 22, 2007
  2. jcsd
  3. Oct 22, 2007 #2


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    Science Advisor

    If dr/dt is any constant other than 0, yes, r is a variable and so dA/dt is a variable. Don't hate r- it's harmless.
  4. Oct 22, 2007 #3
    Ah, thank you once again HallsofIvy.
  5. Oct 22, 2007 #4
    Hmm, one more question. Where does time t come into this? Specifically, it seems that r must be a function of time t so is r really r(t). That is, if I wanted to graph this as time vs dA/dt. A short confirmation by anyone would be appreciated.
  6. Oct 22, 2007 #5
    Yes, r is a function of t. dr/dt is the derivative of that function with respect to t. Since dr/dt does not equal 0, r(t) is not constant. We know that dr/dt does not equal 0 because the problem says that the radius of the circle is changing with respect to time.
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