Results 1 to 9 of 9

Math Help - Finding definite integral of f(x) but not given a function?

  1. #1
    Newbie
    Joined
    Apr 2009
    Posts
    15

    Finding definite integral of f(x) but not given a function?

    I think my prof is trying to trip me up. Here is the question(sorry, I don't know the codes to make it in the right form):

    Suppose: integrand with b=6 and a=1 of f(x) dx=9, integrand with b=4 and a=6 of f(x) dx=4, then find integrand with b=4 and a=1

    does that make any sense? How do I solve this without being given a function??
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by ceb0196 View Post
    I think my prof is trying to trip me up. Here is the question(sorry, I don't know the codes to make it in the right form):

    Suppose: integrand with b=6 and a=1 of f(x) dx=9, integrand with b=4 and a=6 of f(x) dx=4, then find integrand with b=4 and a=1

    does that make any sense? How do I solve this without being given a function??
    You're given \int_1^6 f(x) \, dx = 9 and \int_4^6 f(x) \, dx = 4.

    Note that \int_1^6 f(x) \, dx  = \int_1^4 f(x) \, dx + \int_4^6 f(x) \, dx \Rightarrow \int_1^4 f(x) \, dx = \int_1^6 f(x) \, dx - \int_4^6 f(x) \, dx.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,639
    Thanks
    1592
    Awards
    1
    Is this your question: \int_1^6 f  = 9\;\& \;\int_6^4 f  = 4\; \Rightarrow \;\int_1^4 f  = ??

    If so, notice that \int_4^6 f  =  - 4\;\& \;\int_1^4 f  + \int_4^6 f  = \int_1^6 f .
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    Apr 2009
    Posts
    15
    Yes, that is the question. Thanks to both for the start; I will try to work it out!
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Apr 2009
    Posts
    15
    I just can't figure out what I am missing. How do I solve without having a function that I can plug F(b) and F(a) into??
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by ceb0196 View Post
    I just can't figure out what I am missing. How do I solve without having a function that I can plug F(b) and F(a) into??
    Read posts #2 and #3 again. Read them carefully. Where is your trouble in plugging in the value of things that you know and solving for the thing that you don't know?
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Newbie
    Joined
    Apr 2009
    Posts
    15
    Is it 13? It can't possibly be that simple...
    Follow Math Help Forum on Facebook and Google+

  8. #8
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,639
    Thanks
    1592
    Awards
    1
    Quote Originally Posted by ceb0196 View Post
    Is it 13? It can't possibly be that simple...
    Oh yes, it is indeed that simple!
    Follow Math Help Forum on Facebook and Google+

  9. #9
    Newbie
    Joined
    Apr 2009
    Posts
    15
    Oh my goodness gracious. Thanks to all of you for helping me fumble through the last week of this class!
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 4
    Last Post: April 13th 2011, 02:08 AM
  2. Finding a definite integral
    Posted in the Calculus Forum
    Replies: 2
    Last Post: May 8th 2010, 06:32 AM
  3. definite integral....finding the function
    Posted in the Calculus Forum
    Replies: 1
    Last Post: November 25th 2009, 01:58 PM
  4. finding f for a definite integral
    Posted in the Calculus Forum
    Replies: 3
    Last Post: September 20th 2009, 11:04 AM
  5. finding the derivative of a definite integral
    Posted in the Calculus Forum
    Replies: 3
    Last Post: October 3rd 2008, 02:34 PM

Search Tags


/mathhelpforum @mathhelpforum