|
Author |
Message |
|
TAGS:
|
|
|
Manager
Joined: 05 Aug 2008
Posts: 93
Schools: McCombs Class of 2012
Followers: 1
Kudos [?]:
6
[0], given: 0
|
Square root addition [#permalink]
 02 Jan 2009, 18:42
Question Stats:
64% (02:44) correct
35% (00:33) wrong based on 7 sessions
Attachment: 
Q1.JPG [ 10.17 KiB | Viewed 2385 times ]
|
|
|
|
|
|
|
Manager
Joined: 05 Aug 2008
Posts: 93
Schools: McCombs Class of 2012
Followers: 1
Kudos [?]:
6
[0], given: 0
|
Re: Square root addition [#permalink]
02 Jan 2009, 19:16
That's what I answered, but that is not the correct answer according to the GMATprep.
|
|
|
|
|
|
VP
Joined: 05 Jul 2008
Posts: 1442
Followers: 28
Kudos [?]:
152
[0], given: 1
|
Re: Square root addition [#permalink]
03 Jan 2009, 16:10
9 + sqrt(80) + 9 -sqrt(80) + 2 sqrt (81-80)
=20
|
|
|
|
|
|
Manager
Joined: 05 Aug 2008
Posts: 93
Schools: McCombs Class of 2012
Followers: 1
Kudos [?]:
6
[0], given: 0
|
Re: Square root addition [#permalink]
03 Jan 2009, 21:55
icandy, that is the correct answer, can you please explain your approach?
thanks
|
|
|
|
|
|
Intern
Joined: 30 Sep 2008
Posts: 37
Followers: 0
Kudos [?]:
0
[0], given: 1
|
Re: Square root addition [#permalink]
04 Jan 2009, 18:54
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
Joined: 04 Jan 2009
Posts: 244
Followers: 1
Kudos [?]:
7
[0], given: 0
|
Re: Square root addition [#permalink]
05 Jan 2009, 17:55
(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.
|
|
|
|
|
|
Intern
Joined: 22 Nov 2006
Posts: 12
Followers: 0
Kudos [?]:
0
[0], given: 0
|
Re: Square root addition [#permalink]
10 Jan 2009, 11:46
squaring the term gives
9 + sqrt(80) + 9 - sqrt(80 + 2*(81 - 80) = 20
|
|
|
|
|
|
Intern
Joined: 26 Aug 2009
Posts: 23
Followers: 0
Kudos [?]:
3
[0], given: 3
|
Re: Square root addition [#permalink]
31 Aug 2009, 05:27
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
Joined: 25 Aug 2009
Posts: 190
Followers: 1
Kudos [?]:
47
[0], given: 12
|
Re: Squares and Roots - PS [#permalink]
31 Aug 2009, 15:54
(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
|
|
|
|
|
|
Manager
Joined: 18 Jul 2009
Posts: 174
Location: India
Schools: South Asian B-schools
Followers: 2
Kudos [?]:
39
[1] , given: 37
|
Re: Squares and Roots - PS [#permalink]
31 Aug 2009, 23:10
1
This post received
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
_________________
Bhushan S.
If you like my post....Consider it for Kudos 
|
|
|
|
|
|
Intern
Status: Applying
Joined: 30 Jun 2010
Posts: 34
Concentration: International Business, General Management
GMAT 1: 670 Q V
GMAT 2: 740 Q49 V42
WE: Corporate Finance (Investment Banking)
Followers: 0
Kudos [?]:
8
[0], given: 2
|
Re: Squares and Roots - PS [#permalink]
15 Sep 2010, 00:30
im still not seeing the 81 and 80 here..... 
_________________
"Ignorance more frequently begets confidence than does knowledge" Darwin
|
|
|
|
|
|
GMAT Club team member
Joined: 02 Sep 2009
Posts: 11611
Followers: 1802
Kudos [?]:
9600
[1] , given: 829
|
Re: Squares and Roots - PS [#permalink]
15 Sep 2010, 00:36
1
This post received
KUDOS
|
|
|
|
|
|
Manager
Joined: 22 Jul 2010
Posts: 138
Schools: Wharton,Insead,LBS,IMD,Kellog,Haas,Duke
Followers: 1
Kudos [?]:
6
[0], given: 13
|
Re: Squares and Roots - PS [#permalink]
15 Sep 2010, 02:26
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. Answer: E. Hope it helps. thnxs for the lucid explaination
_________________
Whatever you do, Do it SINCERELY!!!
GOD help those who help themselves....
|
|
|
|
|
|
|
Re: Squares and Roots - PS
[#permalink]
15 Sep 2010, 02:26
|
|
|
|
|
|
|
|
|
|
|
close
