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Integration of Inverse of f(x)

  1. Jan 13, 2008 #1
    [SOLVED] Integration of Inverse of f(x)

    [tex]\int\frac{1}{y^{4}-6y^{3}+5y^{2}}[/tex] dy =


    is this correct way or doing it? thanks. i was a bit of confuse, if i should do it the above or partial fraction the denominator it? thanks
  2. jcsd
  3. Jan 13, 2008 #2


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    yes, factorise and then partial fractions is ok
  4. Jan 13, 2008 #3
    wait a minutes,i show my partial fraction. please point out where gone wrong ;) thanks
  5. Jan 13, 2008 #4


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    firstly you can further factorise y^2-6y+5
  6. Jan 13, 2008 #5



    So, by Partial fraction, is this correct?


    and then, by comparing coefficient of y^3, y^2, y.

    answer is A=6/25, B=1/5, C=3/10. D=-87/50.

    but doubt my answer. anyone can point out?thanks
    Last edited: Jan 13, 2008
  7. Jan 13, 2008 #6
    i know, is just that i m slow using teX, please gimme a min.thanks
  8. Jan 13, 2008 #7


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    I can tell you the answer....

    no need use latex for such simple expression

    most likely you have treated the y^2 bit incorrectly
    Last edited: Jan 13, 2008
  9. Jan 13, 2008 #8
    after i substitute A,B,C,and D.

    i can get back my original 1/f(x)
  10. Jan 13, 2008 #9


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    looks ok so far
  11. Jan 13, 2008 #10
    lol, i uses teX as its more tidy, i go n check the y^2 bit now.thanks.
  12. Jan 13, 2008 #11


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    FYI i've got
  13. Jan 13, 2008 #12
    if so, then
    Last edited: Jan 13, 2008
  14. Jan 13, 2008 #13
    OOops,gimme a minute,i recheck :)
  15. Jan 13, 2008 #14
    you are right! :D :D :D thanks a lot.for pointing it out !:D
  16. Jan 13, 2008 #15
    [tex]\int\frac{1}{y^{4}-6y^{3}+5y^{2}}[/tex] dy =

    6/25(ln|y|)-1/4(ln|y-1|)+1/100(ln|y-5|)-1/(5y). right?

    but this problem is actually make y the subject in this equation, how to go about it further?
    [tex]\int\frac{1}{y^{4}-6y^{3}+5y^{2}}[/tex] dy = [tex]\int\[/tex] dt

    the original question is [tex]\frac{dy}{dt}[/tex]=[tex]y^{4}-6y^{3}+5y^{2}[/tex] find y(t).

    if [tex]\int\frac{1}{y^{4}-6y^{3}+5y^{2}}[/tex] dy =

    then i m stuck to make y the subject. anyone to point out the mistake.thanks
    Last edited: Jan 13, 2008
  17. Jan 13, 2008 #16
    anyone, can show me the right way?
  18. Jan 13, 2008 #17
    another differential eq. question.

    y'=1+xy find y. how to go about it. i m clueless
    my method of multiply dx to (1+xy) doesn't work in this case.
  19. Jan 13, 2008 #18
    You can solve this by the solution for Linear Differential equation. The equation you have is:

    \frac{dy}{dx} = 1 + xy

    which can be written as:

    \frac{dy}{dx} + y(-x) = 1

    which is similar to:

    \frac{dy}{dx} + yP = Q

    Here, [itex]P = (-x); Q = 1[/itex]

    So, you have:

    I.F = e^{\int (-x)dx}
    I.F = e^{-\frac{x^2}{2}}

    And hence, your solution is given by:

    ye^{\frac{-x^2}{2}} = \int (1)e^{\frac{-x^2}{2}}dx + C

    But, it's gonna be a real pain with this:

    \int e^{\frac{-x^2}{2}}dx

    a method to which i can't think of right now. Maybe there's some other way..
    Last edited: Jan 13, 2008
  20. Jan 13, 2008 #19

    Gib Z

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    That integral evaluates to the error function. No "method" for that one, you just have to know it. Haven't checked the rest of your post though, so not sure if the rest of your solution is right.
  21. Jan 14, 2008 #20
    great, this is something new to me, just started my linear algebra course.thanks.will study your solution
  22. Jan 14, 2008 #21


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    sorry, I shouldn't have marked your post as "solved" so early yesterday.
  23. Jan 14, 2008 #22
    thank for pointing it out. ;)
  24. Jan 14, 2008 #23
    is alright, i was thinking of tackle the question part by part. tomorrow is my tutorial lesson, will find out more with tutor. thanks
  25. Jan 14, 2008 #24
    np. however, as i've seen in your posts... try not to make consecutive posts in a discussion. i.e. try to put all the thing u wanna say once in one post only. When someone replies to that post, then go ahead and make a new post in reply. This just keeps the forum clean. No offense.
  26. Jan 14, 2008 #25
    i see, thanks for the advise.will take note.thanks for your effort in solving it as well
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