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# Squares and Roots - PS

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Manager
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00:00

Difficulty:

5% (low)

Question Stats:

71% (02:26) correct 28% (00:28) wrong based on 21 sessions
Attachment:

Q1.JPG [ 10.17 KiB | Viewed 2850 times ]
Manager
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That's what I answered, but that is not the correct answer according to the GMATprep.
VP
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9 + sqrt(80) + 9 -sqrt(80) + 2 sqrt (81-80)

=20
Manager
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thanks
Intern
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These are the concept that you need to know to solve this problem

(x+y)^2 = x^2 +2xy+y^2
sqrtX times sqrt Y = sqrt XY
(a-b)(a+b) = a^2-b^2
Manager
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(x+y)^2 = x^2+y^2+2xy
2*(9+sqrt(80))*(9-sqrt(80)=2*(81-80).

Rest of the explanation is clear from icandy.
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squaring the term gives

9 + sqrt(80) + 9 - sqrt(80 + 2*(81 - 80) = 20
Intern
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nagk78 wrote:
squaring the term gives

9 + sqrt(80) + 9 - sqrt(80 + 2*(81 - 80) = 20

I understand the concept of x^2 + y^2 +2xy ...

But can someone please explain how in the part 2xy they are getting 2*(81-80)?

Because 2xy is 2 * (sqrt(9) + sqrt(80) * sqrt(9) - sqrt(80)) ... or 2 * (sqrt(81)-sqrt(80))

i.e. not simply 81-80

**or is this one of those "find the closest approximation" questions.... and therefore should be spelled out in the question itself on the real GMAT?
Manager
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Re: Squares and Roots - PS [#permalink]  31 Aug 2009, 14:54
(sqrt{a} + sqrt{b})^2 = a + b + 2*sqrt{ab}

Here,
a = 9 + sqrt{81}
b = 9 - sqrt{81}

therefore,
answer = 9 + [strike]sqrt{81}[/strike] + 9 - [strike]sqrt{81}[/strike] + 2*(81-80)
= 9 + 9 + 2 = 20
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Re: Squares and Roots - PS [#permalink]  31 Aug 2009, 22:10
1
KUDOS
i simply approximated

i know .... 4 sqrt(5) = 4 * 2.2 ~ 8.8

on simplyfying (a+b)^ 2 .....i get major terms as 2*9+2*...... = hence ans is more than 18 hence 20
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Re: Squares and Roots - PS [#permalink]  14 Sep 2010, 23:30
im still not seeing the 81 and 80 here.....
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Re: Squares and Roots - PS [#permalink]  14 Sep 2010, 23:36
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Expert's post
djslobo wrote:
im still not seeing the 81 and 80 here.....

Hi and welcome to Gmat Club. Below is properly formated solution for this problem:

You should know two properties:
1. (x+y)^2=x^2+2xy+y^2, ((x-y)^2=x^2-2xy+y^2);

2. (x+y)(x-y)=x^2-y^2.

(\sqrt{9+sqrt{80}}+\sqrt{9-\sqrt{80}})^2=(\sqrt{9+\sqrt{80}})^2+2(\sqrt{9+\sqrt{80}})(\sqrt{9-sqrt{80}})+(\sqrt{9-\sqrt{80}})^2=
=9+\sqrt{80}+2\sqrt{(9+\sqrt{80})(9-\sqrt{80})}+9-\sqrt{80}=18+2\sqrt{9^2-(\sqrt{80})^2}=18+2\sqrt{81-80}=18+2=20.

Hope it helps.
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Re: Squares and Roots - PS [#permalink]  15 Sep 2010, 01:26
Bunuel wrote:
djslobo wrote:
im still not seeing the 81 and 80 here.....

Hi and welcome to Gmat Club. Below is properly formated solution for this problem:

You should know two properties:
1. (x+y)^2=x^2+2xy+y^2, ((x-y)^2=x^2-2xy+y^2);

2. (x+y)(x-y)=x^2-y^2.

(\sqrt{9+sqrt{80}}+\sqrt{9-\sqrt{80}})^2=(\sqrt{9+\sqrt{80}})^2+2(\sqrt{9+\sqrt{80}})(\sqrt{9-sqrt{80}})+(\sqrt{9-\sqrt{80}})^2=
=9+\sqrt{80}+2\sqrt{(9+\sqrt{80})(9-\sqrt{80})}+9-\sqrt{80}=18+2\sqrt{9^2-(\sqrt{80})^2}=18+2\sqrt{81-80}=18+2=20.

Hope it helps.

thnxs for the lucid explaination
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Re: Squares and Roots - PS   [#permalink] 15 Sep 2010, 01:26
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