# Can't find derivative.

• Aug 6th 2012, 04:19 AM
viper483
Can't find derivative.
Can somebody tell me how to find the rate of change to this?

p = 240(1 - [3 / {3 + e^(-0.0005x)}]

I need to find the rate of change when x = 1000.

Thanks
• Aug 6th 2012, 04:40 AM
Prove It
Re: Can't find derivative.
Quote:

Originally Posted by viper483
Can somebody tell me how to find the rate of change to this?

p = 240(1 - [3 / {3 + e^(-0.0005x)}]

I need to find the rate of change when x = 1000.

Thanks

Just so we're clear, is this your function?

\displaystyle \displaystyle \begin{align*} p = 240\left(1 - \frac{3}{3 + e^{-0.0005x}}\right) \end{align*}
• Aug 6th 2012, 04:41 AM
viper483
Re: Can't find derivative.
Quote:

Originally Posted by Prove It
Just so we're clear, is this your function?

\displaystyle \displaystyle \begin{align*} p = 240\left(1 - \frac{3}{3 + e^{-0.0005x}}\right) \end{align*}

Yes, that's correct. I need to learn how to set it out like that!
• Aug 6th 2012, 04:53 AM
Prove It
Re: Can't find derivative.
Quote:

Originally Posted by viper483
Yes, that's correct. I need to learn how to set it out like that!

\displaystyle \displaystyle \begin{align*} p &= 240\left(1 - \frac{3}{3 + e^{-0.0005x}}\right) \\ &= 240 - \frac{720}{3 + e^{-0.0005x}} \\ &= 240 - 720\left(3 + e^{-0.0005x}\right)^{-1} \end{align*}

You should be able to apply the Chain Rule now.
• Aug 6th 2012, 05:18 AM
viper483
Re: Can't find derivative.
Quote:

Originally Posted by Prove It
\displaystyle \displaystyle \begin{align*} p &= 240\left(1 - \frac{3}{3 + e^{-0.0005x}}\right) \\ &= 240 - \frac{720}{3 + e^{-0.0005x}} \\ &= 240 - 720\left(3 + e^{-0.0005x}\right)^{-1} \end{align*}

You should be able to apply the Chain Rule now.

Am I supposed to use the chain rule with the exponent of e as in f(p)=-0.0005x and use that as the inner function?

My teacher said that when using the chain rule with an exponential function like this we're supposed to use the equation d/dx(e^f(x)) = e^f(x) * f '(x).

I'm trying to use that for this equation but I don't think I'm supposed to because the answer doesn't make sense.
• Aug 6th 2012, 05:34 AM
HallsofIvy
Re: Can't find derivative.
There is no way for anyone to know where your answer makes sense or not if you don't tell us what answer you got!
• Aug 6th 2012, 05:58 AM
Prove It
Re: Can't find derivative.
Quote:

Originally Posted by viper483
Am I supposed to use the chain rule with the exponent of e as in f(p)=-0.0005x and use that as the inner function?

My teacher said that when using the chain rule with an exponential function like this we're supposed to use the equation d/dx(e^f(x)) = e^f(x) * f '(x).

I'm trying to use that for this equation but I don't think I'm supposed to because the answer doesn't make sense.

I mean you let \displaystyle \displaystyle \begin{align*} u = 3 + e^{-0.0005x} \end{align*} which gives \displaystyle \displaystyle \begin{align*} y = 240 - 720u^{-1} \end{align*}

You should be able to evaluate \displaystyle \displaystyle \begin{align*} \frac{du}{dx} \end{align*} and \displaystyle \displaystyle \begin{align*} \frac{dy}{du} \end{align*} then multiply them together...
• Aug 6th 2012, 03:26 PM
viper483
Re: Can't find derivative.
Quote:

Originally Posted by Prove It
I mean you let \displaystyle \displaystyle \begin{align*} u = 3 + e^{-0.0005x} \end{align*} which gives \displaystyle \displaystyle \begin{align*} y = 240 - 720u^{-1} \end{align*}

You should be able to evaluate \displaystyle \displaystyle \begin{align*} \frac{du}{dx} \end{align*} and \displaystyle \displaystyle \begin{align*} \frac{dy}{du} \end{align*} then multiply them together...

Sorry, it was midnight here last night, so ended up going to bed.

Ok, so if it's \displaystyle \displaystyle \begin{align*} \frac{du}{dx} \end{align*} then

$\displaystyle u(1000)=3 + e^{-0.0005{1000}}$ which is $\displaystyle 3.60653066...$

Using \displaystyle \displaystyle \begin{align*} \frac{dy}{du} \end{align*} it has to be

$\displaystyle y(u)=240 - 720 (3.60653066)^{-1}$
$\displaystyle = 240 - 720 (0.277274781)$
$\displaystyle =40.36$

Is this correct? The rate of change I should be getting is -0.0168?