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

Question

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

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

rahul16singh28 · Answer

f(2x-1) = 4(x+1)(x-2) ...... When  x = 3, f(5) = 16.

PKN · Answer

When x=3, 2x-1=5

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

Ans. (A)

varun4s · Answer

f(2x-1)= f(5); when x=3

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

Answer: A.

chetan2u · Answer

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

manjot123 · Answer

Equate 2x-1 = 5 you get x=3 
Put x=3 36-8-12
16

Posted from my mobile device

Princ · Answer

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

KanishkM · Answer

to find f(5), never go ahead and put the value as 5 in f(2x-1)

Think how can i get a f(5), when i substitute x = 3

f(5) =36 - 20 
= 16

A

BrentGMATPrepNow · Answer

Great responses above!! Here's one more approach:
 
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 ) = something ² - 9

In general, we can write   f( x ) =  x ² - 9

We're asked to evaluate f( 5 )
So f( 5 ) =  5 ² - 9 = 25 - 9 = 16

Answer: A

Cheers,
Brent

dhruva09 · Answer

Bunuel  Pls format 4x2 to 4x^2 in the question.

Bunuel · Answer

Done. Thank you for noticing!

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

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

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