Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Problem - find second derivative

  1. Dec 15, 2012 #1
    The problem and my attempt at a solution is shown in the attached image.

    The problem is that I end up with one extra x in the denominator.
    So the question is: Is my expression for y'' correct and I just made a mistake somewhere (I have checked it several times), or am I missing something in the expression for y' and/or y'' ?

    attachment.php?attachmentid=53971&stc=1&d=1355635814.png
     

    Attached Files:

  2. jcsd
  3. Dec 16, 2012 #2

    MarneMath

    User Avatar
    Education Advisor

    If the answer in the book is something like 6/x^3 + 18/x^4 + 12/x^5, you got the same answer.
     
  4. Dec 16, 2012 #3

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    How do you go from [itex]\displaystyle\ \frac{6}{x^3}\left( 1+\frac{3}{x}+\frac{2}{x^2}\right)\ [/itex] to [itex]\displaystyle\ \frac{6}{x^4}\left( 1+\frac{1}{x}\right)\left( 1+\frac{2}{x}\right)\ ? [/itex]

    [itex]\displaystyle\ 1+\frac{3}{x}+\frac{2}{x^2}=\left( 1+\frac{1}{x}\right)\left( 1+\frac{2}{x}\right)\ . [/itex]
     
  5. Dec 16, 2012 #4

    symbolipoint

    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    Does this make sense?
    [tex] \[
    \begin{array}{l}
    y' = 3(1 + {\textstyle{1 \over x}})^2 x^{ - 2} ( - 1) \\
    = \frac{{ - 3(1 + {\textstyle{1 \over x}})^2 }}{{x^2 }} \\
    \end{array}
    \]
    [/tex]

    Next you would use quotient rule to find the next derivative (second derivative).
     
  6. Dec 16, 2012 #5
     
  7. Dec 16, 2012 #6

    Curious3141

    User Avatar
    Homework Helper

    Personally, I'd just expand the binomial then differentiate twice. Keep the exponents as negative powers, it makes everything quicker.
     
  8. Dec 16, 2012 #7
    >>Personally, I'd just expand the binomial then differentiate twice
    That certainly makes it much easier. I guess I didnt think of that since I was doing problems related to the chain rule. Thank you for your suggestion.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook