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: MacLaurin Series problem

  1. May 12, 2010 #1
    1. The problem statement, all variables and given/known data
    Find the MacLaurin series of f(x) = ((e^x) - cos (x)) / x

    2. Relevant equations
    e^x = (x^n)/n!
    cos x = ((-1)^(n) (x^(2n))) / (2n)!

    3. The attempt at a solution
    I'm just working on the numerator now, but I can't figure out how to subtract the MacLaurin series listed above to begin the problem. I did find a previous thread in this forum for the same problem, but it talked about if n was odd the cox x series would equal 0 and this doesn't make any sense to me. Can anybody help getting me started?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 12, 2010 #2


    Staff: Mentor

    As written, your "relevant" equations aren't relevant or even true. Assuming you aren't familar with using LaTeX, here are the correct versions. You can click them to see my LaTeX script.

    [tex]e^x = \sum_{n = 0}^{\infty} \frac{x^n}{n!}[/tex]
    [tex]cos(x) = \sum_{n = 0}^{\infty} \frac{(-1)^n x^{2n}}{(2n)!}[/tex]
    Doesn't make any sense to me either.
    Write your series for e^x and cos(x) in expanded form, combine like terms, then divide term-by-term by x.
  4. May 12, 2010 #3
    After your reply I get:

    (1+x+((x^2)/2!)+((x^3)/3!)) - (1-((x^2)/2!)+((x^4)/4!)-((x^6)/6!))

    = (x+((2x^2)/2!)+((x^3)/3!)-((x^4)/4!)+((x^6)/6!)) / x


    Does this seem correct?
  5. May 12, 2010 #4


    Staff: Mentor

    No, for several reasons.
    1. You seem to be very cavalier in your treatment of series. Earlier you wrote that e^x = x^n/n!. Not true. Now you have replaced e^x by 1 + x + x^2/2! + x^3/3!. That's not true either. You need a few more terms, and you need to indicate in some way that both series are infinite series.
    2. The first line is not equal to the second line, so they should not be connected by =. I'll leave it to you to figure out why they're not equal.
    3. Some of the terms are incorrect in your second line.
    4. There are numerous errors in the third line that are unconnected with those in the second line.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook