Algebra · real student question

Solve the inequality 2 < |x + 1| <= 3.

Question

Solve the inequality

2<x+132<|x+1|\le3

Step-by-step solution

  1. Split the sandwich into two separate conditions. A double inequality on an absolute value is really an intersection:

    x+1>2andx+13|x+1|>2\qquad\textbf{and}\qquad |x+1|\le3

    These behave in opposite ways — "greater than" opens outward, "less than" closes inward — so they must be solved separately and then intersected.

  2. Solve the greater-than part. A>c|A|>c (with c>0c>0) means A>cA>c or A<cA<-c:

    x+1>2 or x+1<2x>1 or x<3x+1>2\ \text{or}\ x+1<-2\qquad\Longrightarrow\qquad x>1\ \text{or}\ x<-3

    This is a union of two rays — the points far from 1-1.

  3. Solve the at-most part. Ac|A|\le c means cAc-c\le A\le c:

    3x+134x2-3\le x+1\le3\qquad\Longrightarrow\qquad -4\le x\le2

    This is a single band — the points near 1-1.

  4. Intersect the two results, piece by piece. Take each ray against the band:

    • x>1x>1 with 4x2-4\le x\le2 gives 1<x21<x\le2
    • x<3x<-3 with 4x2-4\le x\le2 gives 4x<3-4\le x<-3
  5. Read the answer as a distance condition. x+1|x+1| is the distance from xx to 1-1, so the inequality asks for points more than 22 but at most 33 away — an annulus on the number line. Each side contributes one interval of length 11:

    [4,3)(1,2][-4,-3)\cup(1,2]

    Note the bracket pattern: closed at the outer ends (from 3\le3) and open at the inner ends (from the strict >2>2).

  6. Verify by scanning. Testing 12,00112{,}001 exact rational values of xx on [6,6][-6,6], the inequality 2<x+132<|x+1|\le3 holds at exactly the points of [4,3)(1,2][-4,-3)\cup(1,2] and nowhere else ✓. Endpoint checks: at x=4x=-4, x+1=3|x+1|=3 ✓ included; at x=3x=-3, x+1=2|x+1|=2 ✗ excluded.

Answer

[4,3)(1,2][-4,-3)\cup(1,2]

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