Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Integral question

  1. Jan 26, 2010 #1
    1. The problem statement, all variables and given/known data

    Evaluate the integral:
    the integral of 9r4^r dr

    2. Relevant equations

    integral of f(x)g'(x)dx = f(x)g(x) - integral of f'(x)g(x)dx

    integral of udv = uv - integral vdu

    u = f(x), dv = g'(x) dx

    3. The attempt at a solution

    I first started by pull the 9 out to the front:
    9 integral of r4^r dr
    I then set u=r, du=dr, dv=4^r, v=(4^r)/ln(4)
    I used the formula: integral of udv = uv - integral vdu
    and got: the integral of 9r4^r dr = [(r4^r)/ln4] - the inegral of (4^r)/ln4 dr
    I then pulled the 1/ln4 out of the last part and got:
    the integral of 9r4^r dr = [(r4^r)/ln4] - 1/ln4[the inegral of 4^r dr]

    I was also able to get the anti-derivative of the [the inegral of 4^r dr] as (1/ln(4)) 4^r
    I got stuck here, i don't know how to put all of it back together, help please?
  2. jcsd
  3. Jan 27, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    You did all the hard stuff already, though you forgot to multiply the 9 through.

    This is what you said you have so far:

    [tex]\int 9r4^rdr=9\left(\frac{r4^r}{\log 4}-\frac{4^r}{(\log 4)^2}\right)[/tex]

    Aren't you done except for tacking on +C to the end?
  4. Jan 27, 2010 #3
    yes, but does it needs to be simplified? how do you do that?
    my assignment is online and it keeps on telling me that i'm getting the wrong answer.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook