Results 1 to 7 of 7

Math Help - Integrating ^-1

  1. #1
    Senior Member
    Joined
    Oct 2009
    Posts
    459

    Integrating ^-1

    I am integrating (1/3)(3-x)^-1.

    Is it (1/3)ln(3-x) ?

    It probably isn't.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member General's Avatar
    Joined
    Jan 2010
    From
    Kuwait
    Posts
    562
    Quote Originally Posted by Stuck Man View Post
    I am integrating (1/3)(3-x)^-1.

    Is it (1/3)ln(3-x) ?

    It probably isn't.
    \frac{1}{3}\int \frac{dx}{3-x} dx=-\frac{1}{3} ln|3-x| + C
    You forgot "-" and "C".
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Senior Member
    Joined
    Oct 2009
    Posts
    459
    I'm not sure why you put - at the front.

    4/x integrates to 4lnx doesn't it?
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,621
    Thanks
    426
    Quote Originally Posted by Stuck Man View Post
    I'm not sure why you put - at the front.

    4/x integrates to 4lnx doesn't it?
    yes, but \frac{1}{3-x} integrates to -\ln|3-x| + C , because the derivative of (3-x) is -1
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Senior Member
    Joined
    Oct 2009
    Posts
    459
    I see, thanks.

    Is k sometimes used instead of C?
    Follow Math Help Forum on Facebook and Google+

  6. #6
    -1
    e^(i*pi)'s Avatar
    Joined
    Feb 2009
    From
    West Midlands, England
    Posts
    3,053
    Thanks
    1
    Quote Originally Posted by Stuck Man View Post
    I see, thanks.

    Is k sometimes used instead of C?
    Not usually but it can be-often with logs.

    It usually arises when you have the case of I = ln|x| + C

    because ln(a)+ln(b) = ln(ab) a new constant, k, is introduced such that C = ln(k)

    I = ln|x| + C = ln|x| + ln(k) = ln(k|x|) and k|x| = e^I
    Follow Math Help Forum on Facebook and Google+

  7. #7
    MHF Contributor

    Joined
    Apr 2005
    Posts
    15,528
    Thanks
    1386
    Quote Originally Posted by Stuck Man View Post
    I see, thanks.

    Is k sometimes used instead of C?
    Any letter can be used to represent a constant- as long as you are clear and consistent.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. need help on integrating this
    Posted in the Calculus Forum
    Replies: 3
    Last Post: June 4th 2010, 06:29 AM
  2. Integrating
    Posted in the Calculus Forum
    Replies: 4
    Last Post: June 2nd 2010, 11:57 AM
  3. integrating
    Posted in the Calculus Forum
    Replies: 1
    Last Post: May 31st 2010, 02:18 PM
  4. integrating e
    Posted in the Calculus Forum
    Replies: 3
    Last Post: April 27th 2010, 11:03 PM
  5. Integrating (2x^2 -3)/x(x^2 +4
    Posted in the Calculus Forum
    Replies: 2
    Last Post: March 17th 2010, 06:14 AM

Search Tags


/mathhelpforum @mathhelpforum