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: Tailor series question

  1. Jan 25, 2008 #1
  2. jcsd
  3. Jan 25, 2008 #2

    Gib Z

    User Avatar
    Homework Helper

    Ok first of all, since we are cube rooting, to get O(x^4) the series inside the cube root must be continued up to the 12th power. Also, you have the right series for the thing inside. Just put that into the Taylor Series for the cube root. It might not work though, because of troubles involved with the cube roots series.
  4. Jan 25, 2008 #3
    i tried to build the series for x^1/3
    in order to do that i've built the first derivative
    the 2nd etc..
    but when i put 0 in the derivative i get zeros for each member
    so the series for the cube root is the cube root itself

    so my other idea is the take this function as a whole and make a series
    out of it
    using the derivatives till the 4th power
    which is very long .

    is there any other way
    because the first way is not working
  5. Jan 25, 2008 #4
    Use the series expansion

    [tex](1+u)^\alpha=1+\alpha\, u+\dots+\frac{\alpha\,(\alpha-1)\dots (\alpha-n+1)}{n!}\, u^n+\dots[/tex]
  6. Jan 25, 2008 #5
  7. Jan 25, 2008 #6
    The series expansion of [itex]\sin(2\,x)[/itex] is

    [tex]\sin(2\,x)=2\,x-\frac{2^3}{3!}\,x^3+\frac{2^4}{5!}\,x^5+\dots \quad (1)[/tex]

    and the series expansion of [itex](1+u)^{1/3}[/itex] is


    For the 4th power is enough to plug for [itex]u[/itex] the first two terms of (1)

    [tex]n=2\rightarrow \frac{\alpha\,(\alpha-1)}{2!}[/tex]

    [tex]n=3\rightarrow \frac{\alpha\,(\alpha-1)\,(\alpha-2)}{3!}[/tex]

    [tex]n=4\rightarrow \frac{\alpha\,(\alpha-1)\,(\alpha-2)\,(\alpha-3)}{4!}[/tex]

  8. Jan 26, 2008 #7
    i got to the conclution that it doesnt matter
    what what the length of each series as long as in the end we get
    a series that on the 4th power
    or on the 5th power in which case we delete the 5th power member

    is that true??
  9. Jan 26, 2008 #8

    Gib Z

    User Avatar
    Homework Helper

    Ok basically for now, yes, continue the series for an infinite number of terms, then truncate at the end. With time you will learn short cuts with the Big-Oh notation, but practice makes perfect.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook