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# X^8 - Y^8 =

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SVP
Status: Graduated
Affiliations: HEC
Joined: 28 Sep 2009
Posts: 1637
Concentration: Economics, Finance
GMAT 1: 730 Q48 V44
Followers: 85

Kudos [?]: 478 [0], given: 431

X^8 - Y^8 = [#permalink]  13 Jul 2010, 17:42
Expert's post
1
This post was
BOOKMARKED
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Difficulty:

25% (medium)

Question Stats:

76% (01:47) correct 24% (01:18) wrong based on 55 sessions
X^8 - Y^8 =

A. (X^4 - Y^4)^2
B. (X^4 + Y^4)(X^2 + Y^2)(X + Y)(X - Y)
C. (X^6 + Y^2)(X^2 - Y^6)
D. (X^4 - Y^4)(X^2 - Y^2)(X - Y)(X + Y)
E. (X^2 - Y^2)^4
[Reveal] Spoiler: OA

_________________
Senior Manager
Joined: 08 Jan 2009
Posts: 332
Followers: 2

Kudos [?]: 66 [1] , given: 5

Re: MGMAT Challenge Problem [#permalink]  13 Jul 2010, 18:39
1
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Apply a^2 - b^2 = (a-b)(a+b)

(x^4)^2 - (y^4)^2

( x^4 + y^4) ( x^4 - y^4)

( x^4 + y^4) (x^2+y^2)(x^2-y^2)

( x^4 + y^4) (x^2+y^2)(x+y)(x-y)

So B
Manager
Joined: 03 Jun 2010
Posts: 184
Location: United States (MI)
Concentration: Marketing, General Management
WE: Business Development (Consumer Products)
Followers: 4

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Re: MGMAT Challenge Problem [#permalink]  14 Jul 2010, 04:44
Looks like 500-level question))
Ms. Big Fat Panda
Status: Biting Nails Into Oblivion
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Re: MGMAT Challenge Problem [#permalink]  14 Jul 2010, 05:13
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The solution is just a basic application of the principle that x^2 - y^2 = (x+y)(x-y)

So, when we simplify, we treat x^8 = (x^4)^2 = ((x^2)^2)^2

So we get the following:

x^8 - y^8 = (x^4+y^4)(x^4-y^4) = (x^4+y^4)(x^2+y^2)(x^2 - y^2) =(x^4+y^4)(x^2+y^2)(x+y)(x-y)

So, the answer is B.
SVP
Status: Graduated
Affiliations: HEC
Joined: 28 Sep 2009
Posts: 1637
Concentration: Economics, Finance
GMAT 1: 730 Q48 V44
Followers: 85

Kudos [?]: 478 [0], given: 431

Re: MGMAT Challenge Problem [#permalink]  14 Jul 2010, 05:56
Expert's post
ulm wrote:
Looks like 500-level question))

Agreed. I personally think that it belongs in the 600-level category, but MGMAT considers it a 700-level question. I just wanted to stay as faithful to the question source as possible.

I'll post a harder one to make up for it!

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Manager
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Re: MGMAT Challenge Problem [#permalink]  15 Jul 2010, 23:24
I go with option B

It is definitely not a 700 level problem
SVP
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Re: X^8 - Y^8 = [#permalink]  24 Aug 2014, 16:04
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Director
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Re: X^8 - Y^8 = [#permalink]  24 Aug 2014, 22:31
BM wrote:
X^8 - Y^8 =

A. (X^4 - Y^4)^2
B. (X^4 + Y^4)(X^2 + Y^2)(X + Y)(X - Y)
C. (X^6 + Y^2)(X^2 - Y^6)
D. (X^4 - Y^4)(X^2 - Y^2)(X - Y)(X + Y)
E. (X^2 - Y^2)^4

I solved this is hardly 3 seconds, would tell how?

x^8 - y^8

It has even power(8 = 2^3) & -ve sign in between, so fully expandable to the least power of x & y i.e (x+y)

All +ve powers cannot be expanded, so out of the 5 options, just search for (x-y) & terms with high +ve powers

Only option B fits in

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Re: X^8 - Y^8 =   [#permalink] 24 Aug 2014, 22:31
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# X^8 - Y^8 =

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