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What is the value of a^3-b^3? 1) a^2+ab+b^2=0 2) a-b=3b Many

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What is the value of a^3-b^3? 1) a^2+ab+b^2=0 2) a-b=3b Many [#permalink] New post 24 Dec 2005, 05:56
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What is the value of a^3-b^3?

1) a^2+ab+b^2=0
2) a-b=3b

Many thanks!
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Re: Pls help DS questions a^3-b^3 [#permalink] New post 24 Dec 2005, 06:19
tennis25 wrote:
Hi guys, could you please help as I have no idea how to simplify this DS question? and then answer it!!
What is the value of a^3-b^3?

1) a^2+ab+b^2=0
2) a-b=3b

Many thanks!


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

1. a^2+ab+b^2= 0 --> S= 0 -->suff
2. -->a= 4b --> a^3-b^3= 64b^3- b^3= 63 b^3
we don't know b --> can't calculate S -->insuff

A it is.
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 [#permalink] New post 24 Dec 2005, 06:38
Laxie, you know a rule for factoring the bins?

For example

a^3+b^3=?
or
a^4-b^4=?

Thx
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 [#permalink] New post 24 Dec 2005, 06:40
allabout wrote:
Laxie, you know a rule for factoring the bins?

For example

a^3+b^3=?
or
a^4-b^4=?

Thx


a^3+b^3 = (a+b)(a^2 -ab +b^2)
a^4- b^4= (a^2- b^2) ( a^2+b^2) = (a-b)*(a+b)*(a^2+b^2)
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 [#permalink] New post 24 Dec 2005, 13:09
a^3 - b^3 = (a-b)(a^2+ab+b^2)

from i) we know (a^2+ab+b^2) = 0, a^3-b^3 = 0 --> Suff
ii) states a-b =3b, i.e a =4b, so a^3-b^3 = 63b^3, --> Insuff

I go with A
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Re: Pls help DS questions a^3-b^3 [#permalink] New post 26 Dec 2005, 00:32
tennis25 wrote:
Hi guys, could you please help as I have no idea how to simplify this DS question? and then answer it!!
What is the value of a^3-b^3?

1) a^2+ab+b^2=0
2) a-b=3b

Many thanks!


a^3-b^3 = (a-b)(a^2+ab+b^2)
= (a-b) * 0
= 0 Hence suff.

From Stmt 2 we get a = 4b, so a^3-b^3 = 64b^3-b^3
= 63b^3, b could be any number hence insuff.

So A.
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 [#permalink] New post 01 Jan 2006, 02:57
Thanks so much for your help all. And a very Happy New Year to you all!
  [#permalink] 01 Jan 2006, 02:57
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