# Connected rate of change

• Feb 28th 2010, 01:44 AM
Punch
Connected rate of change
Two variables, x and y, are related by the equation

$y=\frac{3}{4}(\frac{x}{12}-1)^6.$

Given that both x and y vary with time, find the value of y when the rate of change of y is 12 times the rate of change of x.
• Feb 28th 2010, 05:38 AM
skeeter
Quote:

Originally Posted by Punch
Two variables, x and y, are related by the equation

$y=\frac{3}{4}(\frac{x}{12}-1)^6.$

Given that both x and y vary with time, find the value of y when the rate of change of y is 12 times the rate of change of x.

take the derivative w/r to time

use the given condition $\frac{dy}{dt} = 12 \frac{dx}{dt}$

determine the value of x ... then determine y.
• Feb 28th 2010, 05:49 AM
tom@ballooncalculus
And, just in case a picture helps fill out the chain rule...

http://www.ballooncalculus.org/asy/diffChain/rates.png

... where straight continuous lines differentiate downwards (integrate up) with respect to the main variable (in this case time), and the straight dashed line similarly but with respect to the dashed balloon expression (the inner function of the composite which is subject to the chain rule).

Starting then with...

http://www.ballooncalculus.org/asy/d...ratesRatio.png

_________________________________________
Don't integrate - balloontegrate!

Balloon Calculus; standard integrals, derivatives and methods

Balloon Calculus Drawing with LaTeX and Asymptote!
• Feb 28th 2010, 07:02 AM
skeeter
$y = \frac{3}{4}\left(\frac{x}{12} - 1\right)^6$

$\frac{dy}{dt} = \frac{9}{2}\left(\frac{x}{12} - 1\right)^5 \cdot \frac{1}{12} \cdot \frac{dx}{dt}$

$12 \cdot \frac{dx}{dt} = \frac{9}{2}\left(\frac{x}{12} - 1\right)^5 \cdot \frac{1}{12} \cdot \frac{dx}{dt}$

assuming $\frac{dx}{dt} \ne 0$ ...

$12 = \frac{9}{2}\left(\frac{x}{12} - 1\right)^5 \cdot \frac{1}{12}$

$12 = \frac{3}{8}\left(\frac{x}{12} - 1\right)^5$

$32 = \left(\frac{x}{12} - 1\right)^5$

$2^5 = \left(\frac{x}{12} - 1\right)^5$

$x = 36$

calculate the value of y from the original equation.
• Feb 28th 2010, 05:13 PM
Punch
Quote:

Originally Posted by Punch
Two variables, x and y, are related by the equation

$y=\frac{3}{4}(\frac{x}{12}-1)^6.$

Given that both x and y vary with time, find the value of y when the rate of change of y is 12 times the rate of change of x.

So would I be right to say that from "the rate of change of y is 12 times the rate of change of x" $\frac{dy}{dx}=12$?
• Feb 28th 2010, 05:21 PM
skeeter
if $\frac{dy}{dt} = 12 \cdot \frac{dx}{dt}$ , then $\frac{dy}{dx} = 12$ because ...

$\frac{\frac{dy}{dt}}{\frac{dx}{dt}} = \frac{dy}{dx}$
• Mar 1st 2010, 03:39 AM
Punch
I understood this part but... what I was trying to clarify was... From the sentence "find the value of y when the rate of change of y is 12 times the rate of change of x.", What would I be able to say from the sentence?