Porisma

Worked solution

Carry the logarithm domain through a hence inequality

Sketch y=2/(x+1)y=2/(x+1). By sketching another suitable graph, solve 4/(x+1)≤2∣x∣4/(x+1)\le2|x|.

Hence solve 4/(ln⁡x+1)≤2∣ln⁡x∣4/(\ln x+1)\le2|\ln x|.

CJC · Lecture · Example 11

Sketch the hyperbola and add a modulus graph

Divide the inequality by the positive constant 2 to get 2/(x+1)≤∣x∣2/(x+1)\le|x|. Sketch y=2/(x+1), with vertical asymptote x=−1 and horizontal asymptote y=0, and add y=|x|. The crossing input is x=1. Read where the hyperbola is on or below the V: x<−1 or x≥1. The pole −1 is excluded; the crossing 1 is included.

How did this part go?

Substitute the entire logarithmic input

Divide 4/(ln⁡x+1)≤2∣ln⁡x∣4/(\ln x+1)\le2|\ln x| by 2. It has the same form with the old input replaced by ln x, including inside the modulus. Thus ln x<−1 or ln x≥1.

How did this part go?

Keep the lower domain bound and distinguish the endpoints

The logarithm requires x>0, and its denominator excludes ln x=−1. Since ln is increasing, the intervals become 0<x<e−10<x<e^{-1} or x≥ex\ge e. The lower interval must begin at 0, not at−∞; the source’s common-mistake note warns that this bound is often missed. The threshold e⁻¹ is excluded and e is included.

How did this part go?

Compare your sketch

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The source hyperbola and modulus graph retain pole −1, crossing 1 and both hyperbola branches. The logarithm graph retains levels −1,1 and inputs e⁻¹,e, including the strict positive lower domain bound.
The source hyperbola and modulus graph retain pole −1, crossing 1 and both hyperbola branches. The logarithm graph retains levels −1,1 and inputs e⁻¹,e, including the strict positive lower domain bound.
The source hyperbola and modulus graph retain pole −1, crossing 1 and both hyperbola branches. The logarithm graph retains levels −1,1 and inputs e⁻¹,e, including the strict positive lower domain bound.

Common mistakes

Writing only x<e⁻¹ admits nonpositive inputs where ln x is undefined. Keep the modulus bars in the substituted problem.

Read the hint