# Related Rates setup issue

• Feb 23rd 2009, 11:38 AM
tubasoldier
Related Rates setup issue
I'm having issues setting up a related rates problem. Mostly because it is talking about electricity and I'm not all that sure about how to visualize it.

Quote:

The impedance Z(ohms) in a series circuit is related to the resistance R(ohms) and reactance X(ohms) by the equation $\sqrt[]{R^2+X^2}$. If R is increasing at 3 ohms/sec and X is decreasing at 2 ohms/sec, at what rate is Z changing when R=10 ohms and X=20 ohms?
If I could just have some help with setting this up I would be very grateful.
Thanks
• Feb 23rd 2009, 01:32 PM
Soroban
Hello, tubasoldier!

The subject doesn't matter ... whether it's electricity or fermentation of beer.
It's already set up . . . go for it!

Quote:

Given: . $Z \:=\:\sqrt{R^2+X^2},\quad \frac{dR}{dt} \,=\,3,\quad \frac{dX}{dt} \,=\,-2$

Find $\frac{dZ}{dt}$ when $R = 10,\;x = 20$

We have: . $Z \;=\;\left(R^2+X^2\right)^{\frac{1}{2}}$

Differentiate with respect to time: . $\frac{dZ}{dt} \;=\;\frac{1}{2}\left(R^2+X^2\right)^{-\frac{1}{2}}\left(2R\!\cdot\!\frac{dR}{dt} + 2X\!\cdot\!\frac{dX}{dt}\right)$

. . and we have: . $\frac{dZ}{dt} \;=\;\frac{R\!\cdot\!\frac{dR}{dt} + X\!\cdot\!\frac{dX}{dt}}{\sqrt{R^2 + X^2}}$

Now plug in the given values . . .

• Feb 23rd 2009, 01:48 PM
skeeter
Quote:

Originally Posted by Soroban
The subject doesn't matter ... whether it's electricity or fermentation of beer.

I'll drink to that! (Beer)
• Feb 23rd 2009, 03:50 PM
tubasoldier
Quote:

Originally Posted by Soroban
Hello, tubasoldier!

The subject doesn't matter ... whether it's electricity or fermentation of beer.

Thats pretty funny. You are currently king of my world. Thank you good sir!