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Help with 2 integrals

  1. Dec 2, 2009 #1
    1. The problem statement, all variables and given/known data

    How do I do these 2?

    a) x/(x^2+x-6)

    b) 4/(x^2+4x+4)

    2. Relevant equations



    3. The attempt at a solution

    a) Tried quotient rule it gave me -x^2 - 6/(x^2+x+6)^2, I don't think it's right

    b) Tried substitution (because theres another similar question between these two on the paper and that was the way to go for it) Don't think my answers worth mentioning
     
  2. jcsd
  3. Dec 2, 2009 #2

    Dick

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    There's no 'quotient rule' for integrals! Try partial fractions.
     
  4. Dec 2, 2009 #3
    Factor first the denominator then use the partial fractions.
     
  5. Dec 2, 2009 #4
    Ok, Thanks

    I've now got:

    a) 3/5ln(x+3)+2/5ln(x-2)

    b) 2/(x+2)ln(x+2)^2

    Is that right?
     
  6. Dec 2, 2009 #5

    Mark44

    Staff: Mentor

    Part b is not correct. After factoring you have
    [tex]\int \frac{4}{(x + 2)^2}~dx[/tex]

    You can do this with an ordinary substitution. Your answer should NOT have a log in it!
     
  7. Dec 2, 2009 #6

    Dick

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    If you mean (3/5)ln(x+3)+(2/5)ln(x-2), (use more parentheses!) then the first one looks ok. I don't like the looks of the second one. If you had shown your work, I might have been able to tell you where you went wrong.
     
    Last edited: Dec 2, 2009
  8. Dec 3, 2009 #7
    OK, I've tried b) again but this time by using:

    u.v-(integral of)(v.du/dx)

    u=4
    du/dx=0 (That gets rid of the integral part)

    v=-(x+2)^-1
    dv/dx= (x+2)^-2

    so: 4.-(x+2)^-1

    It would be nice if you could show me how to do it your way (using substitution) aswel
     
  9. Dec 3, 2009 #8

    Dick

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    Let u=(x+2). du=dx. Then 4/(x+2)^2 dx=4/u^2 du=4*u^(-2) du. Use the power law integral u^n=u^(n+1)/(n+1).
     
  10. Dec 3, 2009 #9
    Thank you all for your help
     
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