Welcome! Check out our free B-School Guides to learn how you compare with other applicants.
Login or Register

DS weakness - Inequalities - advise- taking GMAT this week

This topic has 1 expert reply and 5 member replies
DS weakness - Inequalities - advise- taking GMAT this week Post Mon Jul 21, 2008 12:09 am
Hi,

I'm particularly weak in the DS questions pertaining to Inequalities. Would any have any last minute piece of advise for me?
Also, I'm taking the test dayafter, so your last minute advise is most welcome:)
Thanks in advance

Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!

GMAT/MBA Expert

Post Mon Jul 21, 2008 1:11 am
swati.arunahuja wrote:
Hi,

I'm particularly weak in the DS questions pertaining to Inequalities. Would any have any last minute piece of advise for me?
Also, I'm taking the test dayafter, so your last minute advise is most welcome:)
Thanks in advance
there are only a couple of things you really have to know about inequalities, but the number of ways in which those things are combined is almost endless.

here are the facts you need to know:

* when you have SIMULTANEOUS INEQUALITIES, here's the simplest guideline to follow:
- ADD only
- MAKE SURE THE INEQUALITY SIGNS FACE THE SAME WAY

for instance, if you are given the inequalities z - 3m > 0 and 4m - z > 0, you can add those in their current state (since both of the inequality signs are "greater than") to yield m > 0.

if you are given, say, a < b and c > d, and you wanted to combine them, you'd simply turn one of them around by multiplying by a negative quantity, usually -1:
-a > -b
c > d
add: c - a > d - b
there are rules you could memorize for subtracting inequalities, but they're a pain - you have to make sure the inequality signs are facing opposite ways, and, worse yet, you have to know which way the sign is supposed to face after you perform the subtraction. that is not friendly - and it produces the same result as multiplying one side by -1 and then adding anyway.

* do not ever forget to FLIP THE INEQUALITY SIGN when you MULTIPLY OR DIVIDE BY A NEGATIVE NUMBER.
also, corollary: if you DON'T KNOW THE SIGN OF A VARIABLE in an inequality, YOU CAN'T DIVIDE OR MULTIPLY BY IT.
this means that you can't "cross multiply" most inequalities, because cross multiplying is really multiplying by both denominators.
see http://www.manhattangmat.com/forums/if-x-does-not-equal-y-t2997.html for a problem of this type, on which i made copious comments about what you can and can't do with inequalities (sorry for the cross posting).

_________________
Ron is a Director of Curriculum Development at Manhattan GMAT. He has been teaching various standardized tests for 20 years.

--

Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi

--

Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.

Yves Saint-Laurent

--

Learn more about ron

Thanked by: JDesai01, Alokgoolya, chendawg
Free Manhattan GMAT online events - The first class of every online Manhattan GMAT course is free. Classes start every week.
Post Mon Jul 21, 2008 7:10 pm
lunarpower wrote:
swati.arunahuja wrote:
Hi,

I'm particularly weak in the DS questions pertaining to Inequalities. Would any have any last minute piece of advise for me?
Also, I'm taking the test dayafter, so your last minute advise is most welcome:)
Thanks in advance
there are only a couple of things you really have to know about inequalities, but the number of ways in which those things are combined is almost endless.

here are the facts you need to know:

* when you have SIMULTANEOUS INEQUALITIES, here's the simplest guideline to follow:
- ADD only
- MAKE SURE THE INEQUALITY SIGNS FACE THE SAME WAY

for instance, if you are given the inequalities z - 3m > 0 and 4m - z > 0, you can add those in their current state (since both of the inequality signs are "greater than") to yield m > 0.

if you are given, say, a < b and c > d, and you wanted to combine them, you'd simply turn one of them around by multiplying by a negative quantity, usually -1:
-a > -b
c > d
add: c - a > d - b
there are rules you could memorize for subtracting inequalities, but they're a pain - you have to make sure the inequality signs are facing opposite ways, and, worse yet, you have to know which way the sign is supposed to face after you perform the subtraction. that is not friendly - and it produces the same result as multiplying one side by -1 and then adding anyway.

* do not ever forget to FLIP THE INEQUALITY SIGN when you MULTIPLY OR DIVIDE BY A NEGATIVE NUMBER.
also, corollary: if you DON'T KNOW THE SIGN OF A VARIABLE in an inequality, YOU CAN'T DIVIDE OR MULTIPLY BY IT.
this means that you can't "cross multiply" most inequalities, because cross multiplying is really multiplying by both denominators.
see http://www.manhattangmat.com/forums/if-x-does-not-equal-y-t2997.html for a problem of this type, on which i made copious comments about what you can and can't do with inequalities (sorry for the cross posting).
Thanks a lot Ron

Post Mon Jul 21, 2008 7:11 pm
lunarpower wrote:
swati.arunahuja wrote:
Hi,

I'm particularly weak in the DS questions pertaining to Inequalities. Would any have any last minute piece of advise for me?
Also, I'm taking the test dayafter, so your last minute advise is most welcome:)
Thanks in advance
there are only a couple of things you really have to know about inequalities, but the number of ways in which those things are combined is almost endless.

here are the facts you need to know:

* when you have SIMULTANEOUS INEQUALITIES, here's the simplest guideline to follow:
- ADD only
- MAKE SURE THE INEQUALITY SIGNS FACE THE SAME WAY

for instance, if you are given the inequalities z - 3m > 0 and 4m - z > 0, you can add those in their current state (since both of the inequality signs are "greater than") to yield m > 0.

if you are given, say, a < b and c > d, and you wanted to combine them, you'd simply turn one of them around by multiplying by a negative quantity, usually -1:
-a > -b
c > d
add: c - a > d - b
there are rules you could memorize for subtracting inequalities, but they're a pain - you have to make sure the inequality signs are facing opposite ways, and, worse yet, you have to know which way the sign is supposed to face after you perform the subtraction. that is not friendly - and it produces the same result as multiplying one side by -1 and then adding anyway.

* do not ever forget to FLIP THE INEQUALITY SIGN when you MULTIPLY OR DIVIDE BY A NEGATIVE NUMBER.
also, corollary: if you DON'T KNOW THE SIGN OF A VARIABLE in an inequality, YOU CAN'T DIVIDE OR MULTIPLY BY IT.
this means that you can't "cross multiply" most inequalities, because cross multiplying is really multiplying by both denominators.
see http://www.manhattangmat.com/forums/if-x-does-not-equal-y-t2997.html for a problem of this type, on which i made copious comments about what you can and can't do with inequalities (sorry for the cross posting).
Thanks a lot Ron

Post Mon Jul 21, 2008 7:18 pm
lunarpower wrote:
swati.arunahuja wrote:
Hi,

I'm particularly weak in the DS questions pertaining to Inequalities. Would any have any last minute piece of advise for me?
Also, I'm taking the test dayafter, so your last minute advise is most welcome:)
Thanks in advance
there are only a couple of things you really have to know about inequalities, but the number of ways in which those things are combined is almost endless.

here are the facts you need to know:

* when you have SIMULTANEOUS INEQUALITIES, here's the simplest guideline to follow:
- ADD only
- MAKE SURE THE INEQUALITY SIGNS FACE THE SAME WAY

for instance, if you are given the inequalities z - 3m > 0 and 4m - z > 0, you can add those in their current state (since both of the inequality signs are "greater than") to yield m > 0.

if you are given, say, a < b and c > d, and you wanted to combine them, you'd simply turn one of them around by multiplying by a negative quantity, usually -1:
-a > -b
c > d
add: c - a > d - b
there are rules you could memorize for subtracting inequalities, but they're a pain - you have to make sure the inequality signs are facing opposite ways, and, worse yet, you have to know which way the sign is supposed to face after you perform the subtraction. that is not friendly - and it produces the same result as multiplying one side by -1 and then adding anyway.

* do not ever forget to FLIP THE INEQUALITY SIGN when you MULTIPLY OR DIVIDE BY A NEGATIVE NUMBER.
also, corollary: if you DON'T KNOW THE SIGN OF A VARIABLE in an inequality, YOU CAN'T DIVIDE OR MULTIPLY BY IT.
this means that you can't "cross multiply" most inequalities, because cross multiplying is really multiplying by both denominators.
see http://www.manhattangmat.com/forums/if-x-does-not-equal-y-t2997.html for a problem of this type, on which i made copious comments about what you can and can't do with inequalities (sorry for the cross posting).
Thanks a lot Ron

JDesai01 Just gettin' started! Default Avatar
Joined
04 Jul 2008
Posted:
22 messages
Post Tue Jul 22, 2008 7:04 pm
lunarpower- this was a enormously helpful post. Do you have other helpful thoughts like this aggregated somewhere? Seriously, thanks a ton, this has really helped how I approach inequality questions.

And thanks to whoever asked the question!

sumithshah Really wants to Beat The GMAT! Default Avatar
Joined
28 May 2008
Posted:
165 messages
Thanked:
8 times
Test Date:
11/14/2008
Target GMAT Score:
700+
GMAT Score:
730
Post Thu Oct 23, 2008 10:03 pm
Ron

Can you throw some light on the rules of subtracting in equalities?

Best Conversation Starters

1 j_shreyans 86 topics
2 aditya8062 41 topics
3 RiyaR 30 topics
4 kamalakarthi 29 topics
5 anksm22 27 topics
See More Top Beat The GMAT Members...

Most Active Experts

1 image description GMATGuruNY

The Princeton Review Teacher

170 posts
2 image description Brent@GMATPrepNow

GMAT Prep Now Teacher

151 posts
3 image description MBAPrepAdvantage

MBAPrepAdvantage

94 posts
4 image description CriticalSquareMBA

Critical Square

60 posts
5 image description Matt@VeritasPrep

Veritas Prep

58 posts
See More Top Beat The GMAT Experts