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Need help finding derivative/related rates

  1. Oct 2, 2012 #1
    1. Assume that x and y are both differentiable functions of t and find the required values of (dy/dt) and (dx/dt).

    Equation ---> xy = 4

    find (dy/dt) when x = 8

    Given (dx/dt) = 10




    2. Relevant equations
    i tried to find the derivative of xy = 4

    3. The attempt at a solution

    x(dy/dt) + y(dx/dt) = 0

    and then i just plugged in the values but that gave me -(10y/8)

    and the answers is -(5/8)
     
  2. jcsd
  3. Oct 2, 2012 #2
    And what is y when xy=4 and x=8?
     
  4. Oct 2, 2012 #3
    what do you mean? :confused:
     
  5. Oct 2, 2012 #4
    Ohh. nvm i got it. thanks!
    but may i ask why would i need to find for y?
     
  6. Oct 2, 2012 #5
    You know the value of y, so why would you leave it into the form you gave when you can equally well just give a number?
     
  7. Oct 2, 2012 #6

    Mark44

    Staff: Mentor

    In this problem they're asking for the value of dy/dy at the moment when x = 8 and dx/dt = 10. With this information you can solve for y in the equation xy = 4, and evaluate dy/dt.
     
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