1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Can someone confirm this U sub is legal?

  1. Sep 12, 2007 #1
    [tex]\int e^{-4x} dx\\ = \int e^x *e^{-3x}[/tex] let u=e^x so du=e^x dx

    so [tex]\int u^{-3} du[/tex]

    =[tex] -\frac{1}{2}e^{-2x}+c[/tex]

  2. jcsd
  3. Sep 12, 2007 #2
    You made a mistake in the first line, however, it is easy to fix the resulting steps. It should be [itex]e^x \times e^{-5x}[/itex]. I think your method works, however, I think the substitution [itex]u = -4x[/itex] is much easier.

    Also, remember that you can check to see if your answer is correct by taking the derivative at the end.
  4. Sep 12, 2007 #3
    [tex]\int e^{-4x} dx\\ = \int e^{-x} *e^{-3x}[/tex]

    you forgot the negative sign on [tex]e^x[/tex].

    But as mattmns above me suggested, just use u = -4x
  5. Sep 12, 2007 #4
    Oh yeah..it's not a power to a power.

    I did it both ways, and yes they are equal,...and u=-4x is easier.

    [tex] -\frac{1}{4}e^{-4x}[/tex]

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook