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mgmat 3

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by resilient » Sat May 03, 2008 12:29 am
If f(x) = ax4 – 4x2 + ax – 3, then f(b) – f(-b) will equal:

0
2ab
2ab4 – 8b2 – 6
-2ab4 + 8b2 + 6
2ab4 – 8b2 + 2ab – 6

qa is b


I am picking numbers here but am not able to get very far.
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Source: — Data Sufficiency |
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by codesnooker » Sat May 03, 2008 1:00 am
Its Very Simple :D

Here is the solution for you...

f(x) = ax^4 - 4x^2 + ax - 3
Therefore,
f(b) = ab^4 - 4b^2 + ab - 3
and
f(-b) = ab^4 - 4b^2 -ab - 3

Now f(b) - f(-b) = 2ab

You can see except ab all the terms would cancel out.

So answer is (B).
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mgmat3

by resilient » Sat May 03, 2008 10:57 am
I follow you up to the last step. f(-b) = ab^4 - 4b^2 -ab - 3 but shouldnt it be
f(-b) = a(-b)^4 - 4(-b)^2 -a(-b) - 3 . WIth all this, I am getting 2ab-6. Thanks for the explanation. I am understanding much better and am stuck on last detail. Was very tired when studying last night!
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Re: mgmat3

by bigfernhead » Sun May 04, 2008 1:55 pm
resilient wrote:I follow you up to the last step. f(-b) = ab^4 - 4b^2 -ab - 3 but shouldnt it be
f(-b) = a(-b)^4 - 4(-b)^2 -a(-b) - 3 . WIth all this, I am getting 2ab-6. Thanks for the explanation. I am understanding much better and am stuck on last detail. Was very tired when studying last night!
Since f(b) – f(-b), you're negating the second half of the equation, and the -3 changes to a +3... and both of the 3 cancels to 0.
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Re: mgmat3

by codesnooker » Sun May 04, 2008 11:48 pm
resilient wrote:I follow you up to the last step. f(-b) = ab^4 - 4b^2 -ab - 3 but shouldnt it be
f(-b) = a(-b)^4 - 4(-b)^2 -a(-b) - 3 . WIth all this, I am getting 2ab-6. Thanks for the explanation. I am understanding much better and am stuck on last detail. Was very tired when studying last night!
Look the original equation is:

f(x) = ax^4 - 4x^2 + ax - 3

So, when you replace x with -b in left hand side, the same you need to do at the right hand side.

f(b) = a(-b)^4 - 4(-b)^2 + a(-b) - 3

But the equation you mentioned is
resilient wrote: f(-b) = a(-b)^4 - 4(-b)^2 -a(-b) - 3 .
Check here you have changed the sign of coefficient of term x. It should be +a instead of -a.

That's why you are getting incorrect answer.

I hope, now it must be clear to you.
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by Jeff@TargetTestPrep » Mon Dec 11, 2017 11:33 am
resilient wrote:If f(x) = ax^4 - 4x^2 + ax - 3, then f(b) - f(-b) will equal:

A. 0
B. 2ab
C. 2ab^4 - 8b^2 - 6
D. -2ab^4 + 8b^2 + 6
E. 2ab^4 - 8b^2 + 2ab - 6
Since f(x) = ax^4 - 4x^2 + ax - 3, we know:

f(b) = ab^4 - 4b^2 + ab - 3 and:

f(-b) = a(-b)^4 - 4(-b)^2 + a(-b) - 3 = ab^4 - 4b^2 - ab - 3 and finally:

f(b) - f(-b) = (ab^4 - 4b^2 + ab - 3) - (ab^4 - 4b^2 - ab - 3) = 2ab

Answer:B

Jeffrey Miller
Head of GMAT Instruction
[email protected]

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