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: Help with Y'=0 wanted

  1. Aug 30, 2012 #1
    1. The problem statement, all variables and given/known data

    I'm gonna solve the equation Y'=0 if Y=x*e-0,4x

    2. Relevant equations

    3. The attempt at a solution

    I can come as far as to Y'=(1-0,4x)*e-0,4x

    Where do I go from here?
    Can i just write (1-0,4x)*e-0,4x=0 ?

    I can solve the easier kinds of these equations, but this one is the hardest of the ones that I have, and I suspect that something like this will show up on a test in the future, so it would be good if I can solve it.

    Anyone can help me in the right direction?
  2. jcsd
  3. Aug 30, 2012 #2
    Since [tex]{e^a}[/tex] is always > 0 for every value of a, then assume your computed y' is correct, you only need to solve the equation: 1-0.4x = 0, as simple as that.
  4. Aug 30, 2012 #3


    Staff: Mentor

    Yes. Now, either 1 - 0.4x = 0 or e-.4x = 0.

    Note that e-.4x ≠ 0 for any real x.

    Edit: Didn't notice that drawar said essentially the same thing.
  5. Aug 30, 2012 #4
    So I only have to solve 1-0,4x=0 and that will be the whole answer for the whole equation?
  6. Aug 30, 2012 #5


    User Avatar
    Gold Member

    Well, the solution to 1-0.4x =0 solves Y'=0.
  7. Aug 30, 2012 #6
    I don't understand the whole thing :confused:

    Can somebody show all steps to solving this one?
    I really need to learn it.
  8. Aug 30, 2012 #7


    Staff: Mentor

    No, we won't do your work for you, but we'll help you do it.
    The problem, apparently, is to find the x value(s) for which f'(x) = 0, where f(x) = xe-.4x. (Changed from your notation of y(x) to f(x).)

    You found f'(x) = (1 - 0.4x)e-.4x

    If f'(x) = 0, then (1 - 0.4x)e-.4x.

    For what x is f'(x) = 0?
  9. Aug 30, 2012 #8
    According to some earlier posts the solution to 1-0.4x =0 solves Y'=0

    If x is 2.5 then it will be 0.

    But what should I do with e-0,4x?
  10. Aug 30, 2012 #9


    Staff: Mentor

    Nothing. As already mentioned, e-0.4x > 0 for all real x.
  11. Aug 30, 2012 #10
    Okay, so the answer to the problem is:


    e-0,4x is always >0

    And this classifies as the correct answer?
  12. Aug 30, 2012 #11


    Staff: Mentor

    You have a lot of cruft in there that is unnecessary. Here is all you need to say:

    If y = x*e-0.4x, then y' = 0 when x = 2.5.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook