Porisma

Worked solution

Use either a difference graph or a denominator sign test

Find the set of values of x satisfying (x+1)/x≥3/(x+2)(x+1)/x\ge3/(x+2).

CJC · Lecture · Example 6

Method 1: graph one difference

Bring the right side to the left: (x+1)/x−3/(x+2)≥0(x+1)/x-3/(x+2)\ge0. Sketch y=(x+1)/x−3/(x+2)=1+1/x−3/(x+2)y=(x+1)/x-3/(x+2)=1+1/x-3/(x+2). It has vertical asymptotes x=−2 and x=0, horizontal asymptote y=1 and no x-intercepts. The first and third branches are positive; the middle branch is negative. Thus {x∈R:x<−2 or x>0}\{x\in\mathbb R:x<-2\text{ or }x>0\}. To decide this inequality, focus on the branch signs and poles; turning-point coordinates are not needed.

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Method 2: reduce to a sign test

The left side is [(x+1)(x+2)−3x]/[x(x+2)]=(x2+3x+2−3x)/[x(x+2)]=(x2+2)/[x(x+2)][(x+1)(x+2)-3x]/[x(x+2)]=(x^2+3x+2-3x)/[x(x+2)]=(x^2+2)/[x(x+2)]. Since x²+2>0 for every real x, require 1/[x(x+2)]≥01/[x(x+2)]\ge0. Signs around −2,0 are +,−,+. The solution set is {x∈R:x<−2 or x>0}\{x\in\mathbb R:x<-2\text{ or }x>0\}. Neither pole is allowed, and equality never occurs because the numerator is positive.

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Compare your sketch

Expand diagram

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The source graph retains all three branches, poles −2 and 0 and horizontal asymptote 1. The sign line has plus, minus, plus with both poles open, selecting x<−2 or x>0.
The source graph retains all three branches, poles −2 and 0 and horizontal asymptote 1. The sign line has plus, minus, plus with both poles open, selecting x<−2 or x>0.
The source graph retains all three branches, poles −2 and 0 and horizontal asymptote 1. The sign line has plus, minus, plus with both poles open, selecting x<−2 or x>0.

Common mistakes

The ≥ sign does not make an undefined denominator value admissible.

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