# Homework Help: 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)

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.