It is currently 24 Sep 2017, 01:47

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# X = ?

Author Message
TAGS:

### Hide Tags

Manager
Joined: 21 Jul 2009
Posts: 242

Kudos [?]: 132 [0], given: 23

Location: New York, NY

### Show Tags

04 Nov 2009, 14:54
00:00

Difficulty:

(N/A)

Question Stats:

60% (00:34) correct 40% (00:10) wrong based on 13 sessions

### HideShow timer Statistics

I'm having some trouble with the formula...
Attachment:

untitled.JPG [ 6.62 KiB | Viewed 2172 times ]

Kudos [?]: 132 [0], given: 23

Intern
Joined: 25 Oct 2009
Posts: 24

Kudos [?]: 41 [1], given: 12

Schools: Wharton, HBS, Stanford

### Show Tags

04 Nov 2009, 15:32
1
KUDOS
Hi,

try the following approach:

1. Divide both sides by $$(2-sqrt5)$$.

Results in $$x=-\frac{1}{(2-sqrt5)}$$

Skim the answers. There is no any that match. So, modify further

2. Multiply two part of fraction by $$(2+sqrt5)$$ and get

$$x=-\frac{(2+sqrt5)}{(2^2-sqrt5^2)}=-\frac{(2+sqrt5)}{(4-5)}=-\frac{(2+sqrt5)}{(-1)}=2+sqrt5$$

_________________

KUDOS if you find it helpful

Kudos [?]: 41 [1], given: 12

Manager
Joined: 21 Jul 2009
Posts: 242

Kudos [?]: 132 [0], given: 23

Location: New York, NY

### Show Tags

11 Nov 2009, 08:09
Any known roots formula that is useful in these types of questions?

Kudos [?]: 132 [0], given: 23

Senior Manager
Joined: 23 May 2008
Posts: 414

Kudos [?]: 455 [0], given: 14

### Show Tags

11 Nov 2009, 09:10
mendelay wrote:
I'm having some trouble with the formula...
Attachment:
untitled.JPG

Hi I dont know about the formula........but for a quick method just plugin the answer choice and see which answer choice result in the desired value i.e x= -1 in this case..

(2+root 5)*(2-root5)
= 2(2-root5)+root5(2-root5)
=4-2root5+2root5-5
= -1 (root5 gets canceled)

So option A is the correct answer choice
I think this is the quickest method that you can even apply during the real test.

Kudos [?]: 455 [0], given: 14

VP
Joined: 05 Mar 2008
Posts: 1469

Kudos [?]: 295 [0], given: 31

### Show Tags

11 Nov 2009, 09:18
mendelay wrote:
Any known roots formula that is useful in these types of questions?

a tip, whenever i see the square root on the bottom always use the same numbers but change the sign

for example: 1/(2 - sqrt3)

multiply by the same numbers but different sign
(2 + sqrt 3) (1) / (2 - sqrt3)(2+sqrt 3)

Kudos [?]: 295 [0], given: 31

Intern
Joined: 03 Oct 2011
Posts: 24

Kudos [?]: 15 [0], given: 7

### Show Tags

23 Oct 2011, 09:07
In case of surds, try to multiply with the conjugate of the given no. If a+sqrt(b) is given multiply with a-sqrt(b) and vice-versa.

That will lead to the formula (a^2)-(b^2) and it can be solved further from here.
As above, 2-sqrt5 is there, multiply by 2+sqrt5. (Just change the sign and multiply). This helps to eliminate the sqrt part.

Kudos [?]: 15 [0], given: 7

Re: X = ?   [#permalink] 23 Oct 2011, 09:07
Display posts from previous: Sort by