OG PS 14

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resilient: Legendary Member; Posts: 789; Joined: Sun May 06, 2007 1:25 amFollowed by: 6 members; Location: Southern California, USA; Thanked: 15 times

by resilient » Sun Feb 10, 2008 5:23 pm

WHICH OF THE FOLLOWING CANNOT BE A VALUE OF 1/X-1?

A. -1
B. 0
C.2/3
D. 1
E.2

QA IS B

hOWEVER ALL ANSWERS DO YIELD SOME NUMBER? THE SOLUTION IS LIMITED? ANY HELP?

Appetite for 700 and I scraped my plate!

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Source: Beat The GMAT — Problem Solving | Sun Feb 10, 2008 5:23 pm

smar83: Senior | Next Rank: 100 Posts; Posts: 91; Joined: Mon Dec 31, 2007 4:54 pm; Location: Houston; Thanked: 1 times

by smar83 » Sun Feb 10, 2008 5:36 pm

Hi,

I had the same question. The key is do not solve for the value of X.
(Note: Do not substitute for X)...

say, 1/x-1 = 0, You get 1=0, that cannot be true. But, if you put 1/x-1 for any value other than 0, you get atleast some value for x.

So, it is true for all values...other than zero. Hence B.

I hope this helps.

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resilient: Legendary Member; Posts: 789; Joined: Sun May 06, 2007 1:25 amFollowed by: 6 members; Location: Southern California, USA; Thanked: 15 times

ok

by resilient » Sun Feb 10, 2008 9:16 pm

if i enter 0 the equation yields an answer of -1, so its still an answer though? I am puzzled!

Appetite for 700 and I scraped my plate!

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codesnooker: Legendary Member; Posts: 543; Joined: Fri Jan 18, 2008 1:01 am; Thanked: 43 times; GMAT Score:580

Re: ok

by codesnooker » Mon Feb 11, 2008 5:29 am

Enginpasa1 wrote:if i enter 0 the equation yields an answer of -1, so its still an answer though? I am puzzled!

Its pretty simple dude and smar83 has explained it nicely. However, for your convenience, I am explaining it again.

The problem is that we have to compute the value of the given equation, not the value of X. So, for this the examiner has give five sample values of the equation and asked us which value is not possible as the answer of the equation.

i.e. the equation will be 1/1-X = Y, it means you need to compute Y.
So, if we substitute Y = 0, that mean, our equation will be

1/1-X = 0,

which implies 1 = (1-X) * 0.

Anything multiplied by 0 is always equals to ZERO.

so, it means 1 = 0, which practically and theoretically also impossible. Hence the value of Y can never be equal to ZERO.

I hope now you get the answer.

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

Re: ok

by Stuart@KaplanGMAT » Mon Feb 11, 2008 4:31 pm

Enginpasa1 wrote:if i enter 0 the equation yields an answer of -1, so its still an answer though? I am puzzled!

The mistake you're making is that you're subbing in 0 as the value for x. However, 0 would be the value of the entire expression, which is impossible (since 1/anything will never = 0; only 0/anything = 0).

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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