Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 23 May 2017, 16:13

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^2+8x+7<0...................basic stuff but intriguining.

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
TAGS:

Hide Tags

Intern
Joined: 23 Nov 2012
Posts: 3
Location: India
Concentration: Marketing, Marketing
GPA: 3.75
WE: Operations (Telecommunications)
Followers: 0

Kudos [?]: 5 [0], given: 1

x^2+8x+7<0...................basic stuff but intriguining. [#permalink]

Show Tags

25 Nov 2012, 01:58
Hi,

My first post on GMAT Club.I have just kick started my prep for GMAT, get always stuck at the inequalities. Need a bit of help.

Q. x^2+8x+7<0

solution is -7<x<-1

i can break down the equation to (x+1)(x+7)<0

post this x+1<0 the x<-1 which is true as per the answer
but if i follow the similar process for x+7<0 the x<-7 which is not true as per the answer.

please clear my basic, when "<" sign is there in question does the inequality equation follow certain rules.

one more similar question

Q x^2+7x+12>0

answer: x<-4 and x>-3

according to me it should be x>-3 and x>-4 hence solution should be x>-4 .................

Director
Status: Done with formalities.. and back..
Joined: 15 Sep 2012
Posts: 641
Location: India
Concentration: Strategy, General Management
Schools: Olin - Wash U - Class of 2015
WE: Information Technology (Computer Software)
Followers: 49

Kudos [?]: 589 [1] , given: 23

Re: x^2+8x+7<0...................basic stuff but intriguining. [#permalink]

Show Tags

25 Nov 2012, 02:07
1
KUDOS
victorywillbemine wrote:
Hi,

My first post on GMAT Club.I have just kick started my prep for GMAT, get always stuck at the inequalities. Need a bit of help.

Q. x^2+8x+7<0
solution is -7<x<-1
i can break down the equation to (x+1)(x+7)<0
post this x+1<0 the x<-1 which is true as per the answer
but if i follow the similar process for x+7<0 the x<-7 which is not true as per the answer.
please clear my basic, when "<" sign is there in question does the inequality equation follow certain rules.
one more similar question
Q x^2+7x+12>0
answer: x<-4 and x>-3
according to me it should be x>-3 and x>-4 hence solution should be x>-4 .................

Welcome to GMAT club.

The rule is once u get the solution you need to ascertain its validity or you need to plug in that solution back in equation and test for validity. for example:

for first equation, once you've reached to (x+1)(x+7)<0
Now you know that for this to be true, one term must be negative and other positive.
say (x+1) is negative => x <-1
but at this time x+7 must be positive, so x+7 > 0 => x>-7
Hence we get the range -7 <x<-1

Again second possibility,
if x+1 is positive then x+7 should be negative or
x+1 >0 => x >-1
and x+7 <0 => x < -7
but it is not possible. Since x can not be greater than -1 and less than -7 at same time.

So you get that the solution obtained first was correct one. and range is -7<x<-1
similarly you can solve the second problem.

Once you are comfortble getting ans like this, then slowly u can move to graphical approach which makes it even easier to get the answer by knowing the concept.

Hope it helps.
_________________

Lets Kudos!!!
Black Friday Debrief

Intern
Joined: 23 Nov 2012
Posts: 3
Location: India
Concentration: Marketing, Marketing
GPA: 3.75
WE: Operations (Telecommunications)
Followers: 0

Kudos [?]: 5 [0], given: 1

Re: x^2+8x+7<0...................basic stuff but intriguining. [#permalink]

Show Tags

25 Nov 2012, 03:22
My very first reply on the post............yippie

You cleared the funda completely............. My first kudo to you!!!!!!
but just to throw more light on the concept. Do you have material on board to supplement the solution more.

For eg if instead of two cases we have three, how shall I go about in both less than and greater than signs
Q. x(x+1)(x+2)<0
Q. x(x+1)(x+2)>0
Q. (x+1)(x+2)(x+3)<0
Q. (x+1)(x+2)(x+3)>0
Director
Status: Done with formalities.. and back..
Joined: 15 Sep 2012
Posts: 641
Location: India
Concentration: Strategy, General Management
Schools: Olin - Wash U - Class of 2015
WE: Information Technology (Computer Software)
Followers: 49

Kudos [?]: 589 [0], given: 23

Re: x^2+8x+7<0...................basic stuff but intriguining. [#permalink]

Show Tags

25 Nov 2012, 03:29
victorywillbemine wrote:
My very first reply on the post............yippie

You cleared the funda completely............. My first kudo to you!!!!!!
but just to throw more light on the concept. Do you have material on board to supplement the solution more.

For eg if instead of two cases we have three, how shall I go about in both less than and greater than signs
Q. x(x+1)(x+2)<0
Q. x(x+1)(x+2)>0
Q. (x+1)(x+2)(x+3)<0
Q. (x+1)(x+2)(x+3)>0

Ha ha.. i love kudos

Yup, graphical approach... Search for inequalities and you could probably stumble upon the theory. I can give u the concept (not the theory) and way to solve in brief.

take any of the examples, say 1.
x(x+1)(x+2)<0

we have 3 decision points. at 0, -1, and -2 clearly from equation.
arrange these numbers in ascending order.
-2 -1 0
Start with P from right and keep adding P and N alternatively.
you get:
N -2 P -1 N 0 P
Here, N and P defines the range for you where ineuality is positive and negative.
For your question inequality is negative.

therefore the range for x can be -1<x <0 or x<-2

Hope it helps.
_________________

Lets Kudos!!!
Black Friday Debrief

Math Expert
Joined: 02 Sep 2009
Posts: 38841
Followers: 7718

Kudos [?]: 105919 [0], given: 11593

Re: x^2+8x+7<0...................basic stuff but intriguining. [#permalink]

Show Tags

25 Nov 2012, 05:38
victorywillbemine wrote:
Hi,

My first post on GMAT Club.I have just kick started my prep for GMAT, get always stuck at the inequalities. Need a bit of help.

Q. x^2+8x+7<0

solution is -7<x<-1

i can break down the equation to (x+1)(x+7)<0

post this x+1<0 the x<-1 which is true as per the answer
but if i follow the similar process for x+7<0 the x<-7 which is not true as per the answer.

please clear my basic, when "<" sign is there in question does the inequality equation follow certain rules.

one more similar question

Q x^2+7x+12>0

answer: x<-4 and x>-3

according to me it should be x>-3 and x>-4 hence solution should be x>-4 .................

Solving inequalities:
x2-4x-94661.html#p731476
inequalities-trick-91482.html
everything-is-less-than-zero-108884.html
xy-plane-71492.html

_________________
Intern
Joined: 23 Nov 2012
Posts: 3
Location: India
Concentration: Marketing, Marketing
GPA: 3.75
WE: Operations (Telecommunications)
Followers: 0

Kudos [?]: 5 [0], given: 1

Re: x^2+8x+7<0...................basic stuff but intriguining. [#permalink]

Show Tags

25 Nov 2012, 11:11
Thanks Vipps & Bunuel............the concept has become much more simpler.
Re: x^2+8x+7<0...................basic stuff but intriguining.   [#permalink] 25 Nov 2012, 11:11
Similar topics Replies Last post
Similar
Topics:
How well should I know the stuff in GMAT Club mathbook? 2 23 Jan 2014, 16:42
Display posts from previous: Sort by

x^2+8x+7<0...................basic stuff but intriguining.

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group and phpBB SEO Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.