1. Not finding help here? Sign up for a free 30min 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!

Trying to solve |2x-1|-|x+5|-3=0

  1. Jan 11, 2016 #1
    1. The problem statement, all variables and given/known data
    trying to solve |2x-1|-|x+5|-3=0:
    2. Relevant equations
    3. The attempt at a solution

    for x>1/2 I got x=9

    for -5<x<1/2 I got x=-7/3

    but for x<-5 I got -(2x-1)-(-(x+5))-3=0 => x=3 (which is incorrect)

    why x=3 is incorrect , or why I shouldn't considerate the case x<-5 ?
     
  2. jcsd
  3. Jan 11, 2016 #2

    Mark44

    Staff: Mentor

    Because for that interval, (-∞, -5), you are assuming that x < -5. Therefore x cannot equal +3.
     
  4. Jan 11, 2016 #3

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    If your assumption of x <= -5 leads to the conclusion that the root is at x = +3 (outside the starting interval), that means that there IS NO ROOT in the interval (-∞,-5). Plot a graph to see why this is so.
     
  5. Jan 12, 2016 #4

    symbolipoint

    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    You should try checking conditions this way although not as efficient of choices:

    2x-1 and x+5 both positive;
    2x-1 and x+5 both negative;
    2x-1 positive and x+5 negative;
    2x-1 negative and x+5 positive.

    Solve the equation in each of the four conditions and check each result.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Trying to solve |2x-1|-|x+5|-3=0
  1. 20% chance, 5 tries (Replies: 9)

  2. 18cos^2x+3cosx-1 = 0 (Replies: 8)

Loading...