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What is the value of a^4 - b^4 ? 1) a^2 - b^2 = 16 2) a+b =

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What is the value of a^4 - b^4 ? 1) a^2 - b^2 = 16 2) a+b = [#permalink] New post 22 Dec 2006, 09:20
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What is the value of a^4 - b^4 ?

1) a^2 - b^2 = 16
2) a+b = 8
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Re: help w/ GMAT Prep DS question [#permalink] New post 22 Dec 2006, 09:57
dorilee6 wrote:
What is the value of a^4 - b^4 ?

1) a^2 - b^2 = 16
2) a+b = 8



Answer should be c

1) a^2 - b^2= (a+b)(a-b)

2) a+b=8.

Using both 1 and 2 we know a-b=2.

using a+b and a-b values , we can find the value for a and b.

so a^4 - b^4 can be determined.
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 [#permalink] New post 22 Dec 2006, 14:14
What is the value of a^4 - b^4 ?

1) a^2 - b^2 = 16
2) a+b = 8

From A we can’t tell what a or b is, could be any numero!
From B again we don’t know what a or b is, could be any numbers 4.5 + 3.5 OR 7 + 1 etc……

Taking both statements together we can get 2 equations and work out the answer

1 = a^2 = 16 + b^2
2 = a = b – 8

(b-8)(b-8) = 16 + b^2

b^2-16b+64 = 16 + b^2

16b = 48

B = 3

Voila!
  [#permalink] 22 Dec 2006, 14:14
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