Apparently easy Chain Rule Problem

  • Thread starter rambo5330
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  • #1
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Homework Statement



F(s) = ( s - [tex]\frac{1}{s^2}[/tex])3

I have to calculate the derivative of this using chain rule everytime i try i get something way different than in the back of the book... my first move is

3( s - [tex]\frac{1}{s^2}[/tex])2 X ( 1 + [tex]\frac{2}{s^3}[/tex])

is this correct? then expand out from here? maybe theres a problem when i expand.. i dont know but any help would be great thanks...
 

Answers and Replies

  • #2
tiny-tim
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hi rambo5330! :smile:
F(s) = ( s - [tex]\frac{1}{s^2}[/tex])3

I have to calculate the derivative of this using chain rule everytime i try i get something way different than in the back of the book... my first move is

3( s - [tex]\frac{1}{s^2}[/tex])2 X ( 1 + [tex]\frac{2}{s^3}[/tex])
looks ok to me …

what do you get when you expand it?
 
  • #3
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Yeah,

[tex]\frac{d}{ds}F(s) = 3(s - \frac{1}{s^2})^2 (1 + \frac{2}{s^3})[/tex]

Seems fine.

Maybe the author expanded the expression, what answer do you have in the back of the book?

_________________

EDIT: Listen, I've expanded it and what I've found was something like this

[tex]\frac{d}{ds}F(s) = 3(\frac{s^3-1}{s^2})^2(\frac{s^3 +2}{s^3}) \Rightarrow 3(\frac{(s^3-1)^2}{s^4})(\frac{s^3 +2}{s^3}) = \frac{3}{s^7}((s^3-1)^2(s^3+2)) [/tex]
 
Last edited:
  • #4
84
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sorry for late response...

the answer in the text is.

[tex]\frac{d}{ds}F(s) = \frac{3( s^9 - 3s^3 + 2)}{s^7}[/tex]

when i expand i end up with something similar to yours but i obviously made an error somewhere i'm going to try again right now... i really don't see how they are arriving at this solution
 
  • #5
84
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so i finally arrived at the solution thanks a bunch.. i justt needed to know if i was wrong right off the bat or if it was in my expansion and ya jeez.. after awhile of work i found where i made my error.. and i arrived at

[tex]\frac{d}{ds}F(s) = 3(s^2 - \frac{3}{s^4} + \frac{2}{s^7})[/tex]

which in then became clear that the book cleared the fractions by multiplying/dividing by s7

pain in the butt
 

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