1. ## Simple Derivative

Hello,

I'm a bit rusty on my calculus and I have this function and when I use the chain rule I get the f'(x) function

I must be doing something wrong though as when x = 9.17, the dy/dx value should be about 16 but it ends up being about .33 something. Can anyone point out where I went wrong? Thanks in advance.

2. Originally Posted by Jimpson
Hello,

I'm a bit rusty on my calculus and I have this function and when I use the chain rule I get the f'(x) function

I must be doing something wrong though as when x = 9.17, the dy/dx value should be about 16 but it ends up being about .33 something. Can anyone point out where I went wrong? Thanks in advance.
how did you get 75 in the top?

obviously you're using the quotient rule incorrectly.

recall:

Quotient Rule:

$\left( \frac {u}{v} \right)' = \frac {vu' - uv'}{v^2}$

3. Originally Posted by Jhevon
how did you get 75 in the top?
150 * (1+49.1e^-.5x)^-1

-150 * (1+49.1e^.5x)^-2 * -.5e^.5x

75e-.5x
(1+49.1e^.5x)^2

If that makes sense.

4. Originally Posted by Jhevon
how did you get 75 in the top?

obviously you're using the quotient rule incorrectly.

recall:

Quotient Rule:

$\left( \frac {u}{v} \right)' = \frac {vu' - uv'}{v^2}$
I thought quotient rules were used for a function on both the top and bottom of the fraction?

5. Originally Posted by Jimpson
150 * (1+49.1e^-.5x)^-1

-150 * (1+49.1e^.5x)^-2 * -.5e^.5x

75e-.5x
(1+49.1e^.5x)^2

If that makes sense.
ok, so you used the product rule. but note that you have the wrong derivative for the inside.

the derivative of 1 + 49.1e^{-0.5x} is -24.55e^{-0.5x}, there's your problem.

remember, you have a constant 49.1 multiplying the e

6. Originally Posted by Jimpson
I thought quotient rules were used for a function on both the top and bottom of the fraction?
well, technically a constant is a function. it will work anyway. you can use the product rule if you wish, i gave you the corrections already

7. Originally Posted by Jhevon
ok, so you used the product rule. but note that you have the wrong derivative for the inside.

the derivative of 1 + 49.1e^{-0.5x} is -24.55e^{-0.5x}, there's your problem.

remember, you have a constant 49.1 multiplying the e
Ahh, of course, thanks a bunch mate!