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What is the value of a^4 – b^4 ?

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What is the value of a^4 – b^4 ?  [#permalink]

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New post 25 Sep 2017, 00:03
00:00
A
B
C
D
E

Difficulty:

  55% (hard)

Question Stats:

50% (01:53) correct 50% (01:59) wrong based on 32 sessions

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Re: What is the value of a^4 – b^4 ?  [#permalink]

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New post Updated on: 25 Sep 2017, 03:12
a^4 - b^4 =( a^2- b^2) (a^2 + b^2)
a^2- b^2 = (a -b)(a+b)
so a^4 - b^4 = (a -b)(a+b)(a^2 + b^2) Stat 1 NS

Stat 2. NS

Ans C

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Originally posted by Ejiroghene on 25 Sep 2017, 00:14.
Last edited by Ejiroghene on 25 Sep 2017, 03:12, edited 1 time in total.
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Re: What is the value of a^4 – b^4 ?  [#permalink]

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New post 25 Sep 2017, 01:32
Ejiroghene wrote:
a^4 - b^4 =( a^2- b^2) (a^2 + b^2)
a^2- b^2 = (a -b)(a+b)
so a^4 - b^4 = (a -b)(a+b)(a^2 + b^2) which is = (a -b)(a+b)^3 = 10. Stat 1 Suff

Stat 2. NS

Ans A

Sent from my SM-G935F using GMAT Club Forum mobile app



Mistake : \(a^4 - b^4 = (a -b)(a+b)(a^2 + b^2)\) is "Not" equal to \((a -b)(a+b)^3\)

=> \(a^4 - b^4 = (a-b)(a+b)(a^2+b^2) = (a-b)(a+b)(a^2+b^2+2ab-2ab) = (a-b)(a+b)[(a+b)^2 -2ab] = (a-b)(a+b)^3 - 2ab(a-b)(a+b)\)

\(= (a-b)(a+b)^3 - 2ab(a^2-b^2)\)

Statement 1 and statement 2 alone are not sufficient to solve it.
By combining 1 and 2 we can calculate value of \(a^4-b^4\) by putting values from 1 and 2 in above mentioned equation

Answer: C
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Re: What is the value of a^4 – b^4 ? &nbs [#permalink] 25 Sep 2017, 01:32
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