Results 1 to 3 of 3

Math Help - u-subsitution.

  1. #1
    Member
    Joined
    Feb 2010
    From
    NYC
    Posts
    98

    u-subsitution.

    I understand how to do it's just I don't understand what a certain part of my homewor kmeans.
    Using the method of u-substitution,

    \(\displaystyle  \int_{2}^{4} (5 x - 7)^{4} \, dx = \int_{a}^{b} f(u) \, du\)

    where

    u = 5x-7 (enter a function of x)
    du = 5 dx (enter a function of x)
    a =2 (enter a number)
    b =4 (enter a number)
    f(u) =? (enter a function of u).

    The value of the original integral is .
    What does f(u) mean? I plug in u for the values of x?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Junior Member bondesan's Avatar
    Joined
    Jul 2010
    From
    Brazil
    Posts
    58
    Zanderist,

    When you have f(x) = (5x-7)^4 its clever to choose 5x-7 = u, because you are left with (u)^4, so f(u) = u^4 and \int u^4 du = \frac{u^5}{5}, which is "more simple" than the integral with respect of x.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Super Member
    Joined
    Mar 2010
    Posts
    715
    Thanks
    2
    Recheck your upper and lower bounds.
    Quote Originally Posted by Zanderist View Post
    I understand how to do it's just I don't understand what a certain part of my homewor kmeans.
    What does f(u) mean? I plug in u for the values of x?
    f(u) is the new integrand, the function which you have obtained in terms of u after the substitution.
    I plug in u for the values of x?
    No, the whole point of the substitution was to get a new function that's easier to integrate.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 4
    Last Post: August 5th 2010, 05:35 AM
  2. Integration by subsitution?
    Posted in the Calculus Forum
    Replies: 12
    Last Post: June 23rd 2010, 04:14 PM
  3. trig subsitution
    Posted in the Calculus Forum
    Replies: 1
    Last Post: May 10th 2009, 04:42 PM
  4. Integration by subsitution
    Posted in the Calculus Forum
    Replies: 6
    Last Post: January 16th 2008, 04:05 PM
  5. Trigonometric Subsitution
    Posted in the Calculus Forum
    Replies: 13
    Last Post: September 11th 2007, 02:25 PM

/mathhelpforum @mathhelpforum