Need help finding derivative/related rates

1. Oct 2, 2012

physics=world

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. Oct 2, 2012

clamtrox

And what is y when xy=4 and x=8?

3. Oct 2, 2012

physics=world

what do you mean?

4. Oct 2, 2012

physics=world

Ohh. nvm i got it. thanks!
but may i ask why would i need to find for y?

5. Oct 2, 2012

clamtrox

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?

6. Oct 2, 2012

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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