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

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CEO
Joined: 11 Sep 2015
Posts: 3237
If f(2x - 1) = 4x² - 4x - 8, then f(5) =  [#permalink]

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12 Aug 2018, 05:52
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3
00:00

Difficulty:

25% (medium)

Question Stats:

69% (00:56) correct 31% (01:26) wrong based on 64 sessions

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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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Director
Joined: 31 Jul 2017
Posts: 505
Location: Malaysia
GMAT 1: 700 Q50 V33
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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) =  [#permalink]

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12 Aug 2018, 05:57
1
GMATPrepNow wrote:
If f(2x - 1) = 4x² - 4x - 8, then f(5) =

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

$$f(2x-1) = 4(x+1)(x-2)$$...... When $$x = 3, f(5) = 16.$$
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Director
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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) =  [#permalink]

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12 Aug 2018, 06:42
GMATPrepNow wrote:
If f(2x - 1) = 4x² - 4x - 8, then f(5) =

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

When x=3, 2x-1=5

So,$$f(2x-1)=f(5)=4*3^2 - 4*3 - 8=36-12-8=16$$.

Ans. (A)
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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) =  [#permalink]

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12 Aug 2018, 07:00
f(2x-1)= f(5); when x=3

Put x=3 in the equation 4$$x^2$$ -4x - 8. The answer is 16.

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

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12 Aug 2018, 07:24
GMATPrepNow wrote:
If f(2x - 1) = 4x² - 4x - 8, then f(5) =

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

Another way.. May help in such questions somewhere else..

If you look at the equation, you can easily convert the equation ...
$$f(2x-1)=4x^2-4x-8=4x^2-4x+1-9=(2x-1)^2-9$$
So it can be literally written as $$f(x)=x^2-9$$,
So f (5)=5^2-9=25-9=16
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Manager
Joined: 29 Jul 2018
Posts: 106
Concentration: Finance, Statistics
Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) =  [#permalink]

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12 Aug 2018, 07:31
Equate 2x-1 = 5 you get x=3
Put x=3 36-8-12
16

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Joined: 22 Feb 2018
Posts: 411
Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) =  [#permalink]

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12 Aug 2018, 07:54
OA: A

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

Trying lengthier method as other methods have been already discussed above

Let $$2x-1 =y ; x =\frac{y+1}{2}$$

$$f(y) = 4(\frac{y+1}{2})^2 -4 (\frac{y+1}{2}) -8 =(y+1)^2-2(y+1)-8 =y^2+1+2y-2y-2-8=y^2-9$$

$$f(y)=y^2-9$$

$$f(5) =5^2-9=25-9=16$$
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Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) = &nbs [#permalink] 12 Aug 2018, 07:54
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