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Finding domain of function (need an explanation for 2 Qs)

  1. Sep 6, 2015 #1
    • Homework problem moved from a technical forum section
    i need help with finding the domain for a function where there is a square root , here is the picture of such questions :

    http://imgur.com/YcqadOe

    YcqadOe.png
    please see the picture, there are two questions in it, 35 and 37
    let's start with question 35, this is how i try to solve it :

    x^2 - 5x > 0

    x (x-5) >0

    x>0 and x-5>0 so the other answer is x>5

    what to do after getting x>0 and x>5, are they even right?


    the real answer is: (-infinity, 0)u(5,infinity)



    pls help!
     
  2. jcsd
  3. Sep 6, 2015 #2
    You are correct.

    You should re-think about what conditions x should satisfy for x(x-5) to be greater than 0.
     
  4. Sep 7, 2015 #3

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    Does "pls" mean "please"? Please avoid "text speak" here.

    Anyway, ##x(x-5) > 0## does NOT imply that ##x>0## and ##x > 5##. (Note that if you know ##x > 5## you already know that ##x > 0##, so you don't need to list it as a separate condition.)

    More precisely: the inequality ##x(x-5) > 0## means that both ##x## and ##x-5## are nonzero and have the same sign. They do not need to be > 0.
     
  5. Sep 7, 2015 #4

    Mark44

    Staff: Mentor

    As Ray points out, "textspeak" isn't permitted at this site.
    From the forum rules (https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/):
     
  6. Sep 7, 2015 #5

    RJLiberator

    User Avatar
    Gold Member

    For question #37, consider what must occur:

    You cannot have a negative under the square root. So we know that p must be 0 or greater than 0.

    Now, use that same philosophy and the condition to fully solve the problem.
     
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