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: Inequality Problem

  1. Oct 9, 2006 #1
    Could someone tell me how to find the solution set for the following, please.

    [tex]\frac{x}{2-x}<4 [/tex]

  2. jcsd
  3. Oct 9, 2006 #2


    User Avatar
    Science Advisor

    A good way to solve non-linear inequalities such as this is to solve the corresponding equation,
    [tex]\frac{x}{2-x}= 4[/itex]
    The function
    [tex]f(x)= \frac{x}{2-x}- 4[/tex]
    is continuous every where except at x= 2 and so can only change sign at x= 2 or where it is equal to 0. In other words, x= 2 and the solution to the equation divide the number line into intervals on which f(x) is always positive or always negative. By checking one point in each interval you can decide which.
  4. Oct 9, 2006 #3
    Start by multiplying both sides by (2-x).

    However you must bear in mind that, depending upon the possible values of x, that the term (2-x) could be either positive or negative.
    And, when you divide an inequality by a negative number, then you change the direction of the inequality symbol.
  5. Oct 9, 2006 #4
    Got it

    Multiply both sides by [tex] (2-x)^2 [/tex] and then factorise to get solution [tex]x \in (- \infty, \frac{8}{5}) \bigcup (2,\infty) [/tex].

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