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If f(2x - 1) = 4x² - 4x - 8, then f(5) =

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GreenlightTestPrep
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If f(2x - 1) = 4x² - 4x - 8, then f(5) = [#permalink] New post   12 Aug 2018, 05:53
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If f(2x - 1) = 4x² - 4x - 8, then f(5) =

A) 16
B) 18
C) 36
D) 48
E) 72
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A
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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) = [#permalink] New post   12 Aug 2018, 06:07
GreenlightTestPrep wrote:
If f(2x - 1) = 4x² - 4x - 8, then f(5) =

A) 16
B) 18
C) 36
D) 48
E) 72



Here,

2x−1=5

or2x=6

or x=3

Now f(5)=4∗32−4∗3−8=36−12−8=36−20=16

Hence option A
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GreenlightTestPrep
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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) = [#permalink] New post   01 Jun 2020, 11:07
GreenlightTestPrep wrote:
If f(2x - 1) = 4x² - 4x - 8, then f(5) =

A) 16
B) 18
C) 36
D) 48
E) 72


First notice that 4x² - 4x - 8 looks a lot like (2x - 1)²

(2x - 1)² = (2x - 1)(2x - 1) = 4x² - 4x + 1

Now take: f(2x - 1) = 4x² - 4x - 8
Rewrite as: f(2x - 1) = 4x² - 4x + 1 - 9 = (2x - 1)² - 9
In other words, f(something) = something² - 9

In general, we can write f(x) = x² - 9

We're asked to evaluate f(5)
So f(5) = 5² - 9 = 25 - 9 = 16

Answer: A

Cheers,
Brent
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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) = [#permalink] New post   16 Jun 2021, 11:14
Hello from the GRE Prep Club BumpBot!

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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) = [#permalink]
16 Jun 2021, 11:14
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