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: Confused . y=x-|x|

  1. Feb 19, 2008 #1
    confused ..... y=x-|x|

    The problem is to graph this equation y=x-|x|.

    From what I understand of absolute values, this x would be positive. If it is positive then y=0 and there would be no points to graph. Is there something that I am missing? The question is worth 4 points, so I can't see the answer just being 0. Thanks for any suggestions.
  2. jcsd
  3. Feb 19, 2008 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    What happens when x is negative?
  4. Feb 19, 2008 #3
    What I was taught to do when dealing with absolute value, is to rewrite the equation so that the absolute value is isolated, then find the 2 equations.

    so you'll have:

    and also:
  5. Feb 19, 2008 #4
    The absolute value of -x would be x. But I remember with inequalities there are 2 possible answers with absolute value (-,+). From what you are explaining it sounds like that is what you are saying to do, use both possible values. Then I would graph by beginning with y at 0 and continue by substituting values into 2x=y? That seems to make sense to me. Thank you both for your help!
  6. Feb 19, 2008 #5
    Also just try plugging in some numbers:
    For 2 -> Y = 2 - |2| = 0
    For -2 - > Y = -2 -|-2| = -2 -2 = -4
    So for negatives you have Y = 2X, X<0
  7. Feb 19, 2008 #6


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    Solve or graph y = x - |x|.

    If x>0, then y = x - x, meaning y=0.

    If x<0, then y = x - (-x) [ notice those are parentheses, not absolute value notation symbols ], meaning y = x + x = 2x.
  8. Feb 19, 2008 #7


    User Avatar
    Science Advisor

    Surely you didn't mean to say that! x itself can be any number. |x| is always positive (or 0- don't forget that!

    Why did you switch to x=? If x[itex]\ge 0[/itex] y= x- x= 0. The graph is just the x axis from x= 0 to the right.

    If x< 0, don't forget that. Then y= x- (-x)= 2x.

    No, no, no! |-x|= |x| which may be eigther x or -x depending upon what x is.

    Draw the graph of y= 2x, to the left of x= 0. To the right, the graph is just y= 0, the x-axis.
  9. Feb 19, 2008 #8
    I rearranged the formula to isolate the absolute value.
  10. Feb 19, 2008 #9
    Wow, you guys couldn't have made it any more clear for me. Thanks alot!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook