Algebra · real student question

Carry out a sign study to solve the inequality (|x - 2| - x)/(x² - 4) ≤ 0.

Question

Solve, by a sign study,

x2xx240.\frac{|x-2|-x}{x^{2}-4}\le 0.

Step-by-step solution

  1. Fix the domain. The denominator factors as x24=(x2)(x+2)x^{2}-4=(x-2)(x+2), so it vanishes at x=±2x=\pm2. Those two points must be excluded:

    D=R{2,2}.D=\mathbb{R}\setminus\{-2,\,2\}.

    They will also be the natural break points of the sign chart.

  2. Remove the absolute value by cases at x=2x=2. For x2x\ge2, x2=x2|x-2|=x-2 and the numerator is

    x2x=2,x-2-x=-2,

    a constant that is always negative. For x<2x<2, x2=2x|x-2|=2-x and the numerator is

    2xx=22x=2(1x),2-x-x=2-2x=2(1-x),

    positive for x<1x<1, zero at x=1x=1, negative for 1<x<21<x<2. So the numerator changes sign only at x=1x=1.

  3. Chart the denominator. (x2)(x+2)(x-2)(x+2) is positive outside [2,2][-2,2] and negative inside:

    x<2:  +2<x<2:  x>2:  +.x<-2:\;+\qquad -2<x<2:\;-\qquad x>2:\;+.

  4. Combine on the four intervals cut by 2-2, 11 and 22.

    • (,2)(-\infty,-2): numerator ++, denominator ++ \Rightarrow quotient ++ — excluded.
    • (2,1)(-2,1): numerator ++, denominator - \Rightarrow quotient - — included.
    • (1,2)(1,2): numerator -, denominator - \Rightarrow quotient ++ — excluded.
    • (2,+)(2,+\infty): numerator 2-2, denominator ++ \Rightarrow quotient - — included.
  5. Handle the boundary points and write the answer. At x=1x=1 the numerator is 00 and the denominator is 30-3\ne0, so the quotient is 00 and the point satisfies 0\le 0; it is included. The points x=±2x=\pm2 are never allowed. Hence

    (2,1]    (2,+).(-2,\,1]\;\cup\;(2,\,+\infty).

    Spot checks: x=0x=0 gives 2/(4)=0.502/(-4)=-0.5\le0 ✓, x=1.1x=1.1 gives 0.2/(2.79)=+0.0717>0-0.2/(-2.79)=+0.0717>0 ✗, x=3x=3 gives 2/5=0.40-2/5=-0.4\le0 ✓, x=3x=-3 gives 8/5=+1.6>08/5=+1.6>0 ✗.

Answer

(2,1](2,+)(-2,\,1]\cup(2,\,+\infty)

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