Source Grockit: DS: Inequallity

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polter: Junior | Next Rank: 30 Posts; Posts: 16; Joined: Mon Dec 28, 2009 3:11 am

by polter » Wed Apr 25, 2012 6:27 pm

If m â‰ -n, is (m-n)/(m+n)> 1?

(1) n < 0

(2) m > 0

[spoiler]E[/spoiler]

I think the correct answer should be different.

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Source: Beat The GMAT — Data Sufficiency | Wed Apr 25, 2012 6:27 pm

shantanu86: Senior | Next Rank: 100 Posts; Posts: 59; Joined: Sun Mar 11, 2012 8:57 pmFollowed by: 1 member; Location: India; Thanked: 16 times

by shantanu86 » Wed Apr 25, 2012 7:39 pm

polter wrote:If m â‰ -n, is (m-n)/(m+n)> 1?

(1) n < 0

(2) m > 0

E

I think the correct answer should be different.

Hi polter,

Pls go th. https://www.beatthegmat.com/fast-way-to- ... tml#467098

Regards,
Shantanu

If you feel like it, hit thanks

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spartacus1412: Master | Next Rank: 500 Posts; Posts: 110; Joined: Wed Feb 22, 2012 11:28 pmFollowed by: 1 member; Location: India; Thanked: 13 times

by spartacus1412 » Wed Apr 25, 2012 7:41 pm

the question says m +n =0 (given m!= -n)
to find if (m-n)/(m+n)> 1?

this can be reduced to (m-n)/(m+n) -1 >0
i.e. -2n/(m+n)>0

option1: n<0
we cannot say anything about the sign of m+n. hence, above expression can be + as well as -
hence, insufficient.

option1: m>0
we cannot say anything about the sign of above expression. hence, above expression can be + as well as -
hence, insufficient.

combine both : n<0 and m>0
the above expression wil be less than zero only if abs(n)> abs(m), which is not given.

hence, both together are insufficient.
Answer, E

Hope it Helps

Its do or die this time!
Practise, practise and practise.

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polter: Junior | Next Rank: 30 Posts; Posts: 16; Joined: Mon Dec 28, 2009 3:11 am

by polter » Sun Apr 29, 2012 10:31 am

Hi shantanu86/ spartacus1412, thank you for your suggestions.

My confusion was with this: can this expression not be reduced as
(m-n)/(m+n)> 1
=> (m-n)> (m+n)
=> -n > n
=> 0 > 2n
=> 0 > n

Since (1) clearly defines this. Hence A

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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

by Stuart@KaplanGMAT » Sun Apr 29, 2012 12:43 pm

polter wrote:Hi shantanu86/ spartacus1412, thank you for your suggestions.

My confusion was with this: can this expression not be reduced as
(m-n)/(m+n)> 1
=> (m-n)> (m+n)
=> -n > n
=> 0 > 2n
=> 0 > n

Since (1) clearly defines this. Hence A

Hi!

Whenever you see inequalities and variables, alarm bells should go off in your head and your internal warning system should be shouting "DANGER DANGER DANGER!!!"

Remember this key difference between equations and inequalities:

When you multiply or divide both sides of an inequality by a negative number, you flip the inequality.

With that rule in mind, here's the mistake you made: you divided both sides by (m+n) without knowing whether that term was positive or negative.

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
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