For which of the following functions does \(f(x) = f\left(\dfrac1{x}\right),\) given that \(x \ne - 2, - 1, 0,\) or 1? - The Beat The GMAT Forum

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For which of the following functions does \(f(x) = f\left(\dfrac1{x}\right),\) given that \(x \ne - 2, - 1, 0,\) or 1?

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Vincen: Legendary Member; Posts: 2898; Joined: Thu Sep 07, 2017 2:49 pm; Thanked: 6 times; Followed by:5 members

For which of the following functions does \(f(x) = f\left(\dfrac1{x}\right),\) given that \(x \ne - 2, - 1, 0,\) or 1?

by Vincen » Wed Sep 30, 2020 6:54 am

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For which of the following functions does \(f(x) = f\left(\dfrac1{x}\right),\) given that \(x \ne - 2, - 1, 0,\) or \(1?\)

A. \(f(x)=\left|\dfrac{x+1}{x}\right|\)

B. \(f(x)=\left|\dfrac{x+1}{x-1}\right|\)

C. \(f(x)=\left|\dfrac{x-1}{x}\right|\)

D. \(f(x)=\left|\dfrac{x}{x+1}\right|\)

E. \(f(x)=\left|\dfrac{x+1}{x+2}\right|\)

Answer: B

Source: Manhattan GMAT

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Source: Beat The GMAT — Problem Solving | Wed Sep 30, 2020 6:54 am

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8086; Joined: Sat Apr 25, 2015 10:56 am; Location: Los Angeles, CA; Thanked: 43 times; Followed by:29 members

Re: For which of the following functions does \(f(x) = f\left(\dfrac1{x}\right),\) given that \(x \ne - 2, - 1, 0,\) or

by Scott@TargetTestPrep » Fri Oct 09, 2020 9:13 am

Vincen wrote: ↑
Wed Sep 30, 2020 6:54 am
For which of the following functions does \(f(x) = f\left(\dfrac1{x}\right),\) given that \(x \ne - 2, - 1, 0,\) or \(1?\)

A. \(f(x)=\left|\dfrac{x+1}{x}\right|\)

B. \(f(x)=\left|\dfrac{x+1}{x-1}\right|\)

C. \(f(x)=\left|\dfrac{x-1}{x}\right|\)

D. \(f(x)=\left|\dfrac{x}{x+1}\right|\)

E. \(f(x)=\left|\dfrac{x+1}{x+2}\right|\)

Answer: B

Solution:

The easiest way to solve the problem (without solving the problem algebraically) is to use a value for x (other than -2, -1, 0, and 1). So, let’s use x = 2.

A) f(2) = |(2 + 1)/2| = 3/2 f(1/2) = |(1/2 + 1)/(1/2)| = (3/2)/(1/2) = 3

Since f(2) ≠ f(1/2), A is not the correct answer.

B) f(2) = |(2 + 1)/(2- 1)| = 3/1 = 3 f(1/2) = |(1/2 + 1)/(1/2 - 1)| = |(3/2)/(-1/2)| = |-3| = 3

Since f(2) = f(1/2), B could be the correct answer so long as we can show that none of the three remaining choices can be the correct answer.

C) f(2) = |(2 - 1)/2| = 1/2 f(1/2) = |(1/2 - 1)/(1/2)| = |(-1/2)/(1/2)| = |-1| = 1

Since f(2) ≠ f(1/2), C is not the correct answer.

D) f(2) = |2/(2 + 1)| = 2/3 f(1/2) = |(1/2)/(1/2 + 1)| = (1/2)/(3/2) = 1/3

Since f(2) ≠ f(1/2), D is not the correct answer.

E) f(2) = |(2 + 1)/(2 + 2)| = 3/4 f(1/2) = |(1/2 + 1)/(1/2 + 2)| = (3/2)/(5/2) = 3/5

Since f(2) ≠ f(1/2), E is not the correct answer.

Answer: B

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