or for the Beat The GMAT Forums

Some questions from GMAT Prep - please help

[This topic has 2 expert replies and 18 member replies]

Forum Index GMAT Math

Free $100 Amazon.com Gift Card - Buy a GMAT course using a Beat The GMAT discount code between Mar 8-22 and get a $100 Amazon.com Gift Card. Learn more!

Goto page 1, 2 Next

« View previous topic

View next topic »

momentary_lapse
Rising GMAT Star

Joined: 26 Feb 2007
Posts: 43

Thanks given: 0
Thanked 0 times in 0 posts

Topic: Some questions from GMAT Prep - please help

Sat Mar 24, 2007 10:39 pm

	Elapsed Time:	00:00		Why a timer is critical to improving your score
	Lap #[LAPCOUNT] ([LAPTIME])

1) If x<0 , what is square root of -x|x|

I thought if x<0, |x| = -x
THerefore square root becomes the root of x^2 which is + or - x

I wrote the answer as x but the GMAT Prep says that answer is -x. How???

2) Data Sufficiency:

Is 1/p > r/(r^2 + 2)

1) p=r
2) r=0

Please help with this too

Thanks a lot in advance!

Send private message

dsuna
Just gettin' started!

Joined: 17 Mar 2007
Posts: 14

Thanks given: 0
Thanked 0 times in 0 posts

Sun Mar 25, 2007 9:58 am

momentary_lapse wrote: 1) If x<0 , what is square root of -x\|x\| I thought if x<0, \|x\| = -x THerefore square root becomes the root of x^2 which is + or - x I wrote the answer as x but the GMAT Prep says that answer is -x. How??? 2) Data Sufficiency: Is 1/p > r/(r^2 + 2) 1) p=r 2) r=0 Please help with this too Thanks a lot in advance! 1.) X<0, let's say x= -2, then square root of -x\|x\| = square root of -(-2)*\|-2\| = square root of -(-2)(-2) =square root of -4 = -2, hence solution is -x 2.) is the answer D? r^2+2 > pr r^2-pr+2>0 1. p=r then r^2-r^2+2>0, 2>0, sufficient 2. r=0 then 0-0+2>0, 2>0, sufficient the answer should be D

Send private message

jayhawk2001
Moderator

Joined: 28 Jan 2007
Posts: 789

Thanks given: 0
Thanked 15 times in 14 posts
Location: Silicon valley, California

Sun Mar 25, 2007 10:55 am

For question 2 -- 1 - insufficient. For p = r = 2, we have 1/2 > 1/3 which is true. For p = r = -2, we have -1/2 > -1/3 which is not true. 2 - insufficient. We just have 1/p > 0 Combining 1 and 2, we have p = r = 0 which leads us nowhere. Is it E ?

Send private message

aim-wsc
GMAT Titan

Joined: 20 Apr 2006
Posts: 2469

Thanks given: 144
Thanked 57 times in 45 posts
Location: BtG Underground

Target GMAT Score: 801-

Sun Mar 25, 2007 1:47 pm

give one more try @DS guys _________________ Getting started @BTG? Beginner's Guide to GMAT \| Johny learns to blog Please do not PM me, (not active anymore) contact Eric.

Send private message

Visit poster's website

momentary_lapse
Rising GMAT Star

Joined: 26 Feb 2007
Posts: 43

Thanks given: 0
Thanked 0 times in 0 posts

Sun Mar 25, 2007 7:31 pm

The answer to 2 is C. The first equation is insufficient since the inequality sign will change based on negative and positive values of r. The second one makes it clear by saying that r is +ve and p=r i.e. the equation will hold good. For the first one, if x = -2 then suare root of -x\|x\| = -(-2)\|-2\| which is = 2\|-2\| = 2*2 = 4 and square root is + or -2 . How can the square root of a negative number be a negative number. It should be imaginery.

Send private message

momentary_lapse
Rising GMAT Star

Joined: 26 Feb 2007
Posts: 43

Thanks given: 0
Thanked 0 times in 0 posts

Sun Mar 25, 2007 7:35 pm

However the GMAT prep gave the options as -x,x,1,0 and square root of x. The answer is also supposed to be -x. How?? I just cant figure it out.

Send private message

aim-wsc
GMAT Titan

Joined: 20 Apr 2006
Posts: 2469

Thanks given: 144
Thanked 57 times in 45 posts
Location: BtG Underground

Target GMAT Score: 801-

Sun Mar 25, 2007 8:10 pm

present the original answer, what GMATprep has to say. and do check the announcement made at sub-forums. Thanks for the patience. explanation is on the way _________________ Getting started @BTG? Beginner's Guide to GMAT \| Johny learns to blog Please do not PM me, (not active anymore) contact Eric.

Send private message

Visit poster's website

jayhawk2001
Moderator

Joined: 28 Jan 2007
Posts: 789

Thanks given: 0
Thanked 15 times in 14 posts
Location: Silicon valley, California

Sun Mar 25, 2007 8:48 pm

[quote="momentary_lapse"]The answer to 2 is C. The first equation is insufficient since the inequality sign will change based on negative and positive values of r. The second one makes it clear by saying that r is +ve and p=r i.e. the equation will hold good. [/quote] Hmm, (2) just leads to 1/p > 0. However, if you combine 1 and 2, you get p = r = 0. One can't cross-multiply this inequality when p = r = 0. 1/p > r/(r^2 + 2) The question hence boils down to the following Is 1/ 0 > 0 / (0 + 2) I'm still not fully convinced that C is the correct answer but I guess GMAT Prep answer is the final answer

Send private message

Neo2000
GMAT Destroyer!

Joined: 27 Jan 2007
Posts: 519

Thanks given: 19
Thanked 29 times in 29 posts
Location: India

Test Date: 30/09
Target GMAT Score: 710

Mon Mar 26, 2007 12:50 am

momentary_lapse wrote: 1) If x<0 , what is square root of -x\|x\| I thought if x<0, \|x\| = -x THerefore square root becomes the root of x^2 which is + or - x You were fine till here. Then since we said in the very beginning that X<0 solution set CANNOT contain +X Therefore your answer is -X

Send private message

Visit poster's website

momentary_lapse
Rising GMAT Star

Joined: 26 Feb 2007
Posts: 43

Thanks given: 0
Thanked 0 times in 0 posts

Mon Mar 26, 2007 9:28 am

Neo, If x = -2 then the square root of X^2 is + or -2 right? Because x^2 is 4 and whether x is < 0 or > 0 the square root of a +ve number can be either the +ve root or the -ve root. What does the value of x have to do with this? Lets say x=-2, then the answer is -x which is -(-2) = 2. This defeats the reasoning you are giving saying that x < 0 therefore the answer should be -x.

Send private message

Neo2000
GMAT Destroyer!

Joined: 27 Jan 2007
Posts: 519

Thanks given: 19
Thanked 29 times in 29 posts
Location: India

Test Date: 30/09
Target GMAT Score: 710

Mon Mar 26, 2007 10:11 am

momentary_lapse wrote: Neo, If x = -2 then the square root of X^2 is + or -2 right? Because x^2 is 4 and whether x is < 0 or > 0 the square root of a +ve number can be either the +ve root or the -ve root. Yup the root can be either positive or negative. However, since we said X is <0 then we cannot have a value of X>0 which means the answer has to be -2 momentary_lapse wrote: What does the value of x have to do with this? Lets say x=-2, then the answer is -x which is -(-2) = 2. This defeats the reasoning you are giving saying that x < 0 therefore the answer should be -x. My reasoning was as follows Modulus function always give the positive value/max value/magnitude of X Since X<0, value of X is negative say -y Then -(-y)\|-y\| = y(y) = y^2 So square root of this becomes +or- y However we've already said that X<0 which means X cannot have a positive value so the value has to be -y

Send private message

Visit poster's website

gabriel
GMAT Destroyer!

Joined: 20 Dec 2006
Posts: 986

Thanks given: 138
Thanked 37 times in 34 posts
Location: India

Mon Mar 26, 2007 10:36 am

jayhawk2001 wrote: Hmm, (2) just leads to 1/p > 0. However, if you combine 1 and 2, you get p = r = 0. One can't cross-multiply this inequality when p = r = 0. 1/p > r/(r^2 + 2) The question hence boils down to the following Is 1/ 0 > 0 / (0 + 2) I'm still not fully convinced that C is the correct answer but I guess GMAT Prep answer is the final answer jay u r right according to the given statements... the answer shuld be E... even if both the staements are combined we wuld get ( as already stated by u ).... 1/0 and 0/ ( 0+2 )... and 1/0 is mathematically not defined.... the second statment is actually r>0.... by using this we get the answer as C...

Send private message

gabriel
GMAT Destroyer!

Joined: 20 Dec 2006
Posts: 986

Thanks given: 138
Thanked 37 times in 34 posts
Location: India

Mon Mar 26, 2007 11:53 am

momentary_lapse wrote: However the GMAT prep gave the options as -x,x,1,0 and square root of x. The answer is also supposed to be -x. How?? I just cant figure it out. Ok.. the q asks to find the sq root ( -x mod (x))= sq root ( -x )* sq root ((mod(x)) = sq root (-x)* sq root(-x)... let this be eqn no. 1.... .. for understanding this u need to know the basics of complex numbers... the root of a negative qty -x can be written as sqroot (x)* sqroot (-1)... so applying this to eqn 1.... we get sqroot (x)sqroot(-1) sqroot(x)sqroot(-1) .... that is sqroot (x)sqroot(x)sqroot(-1)sqroot(-1)... which is equal to -x... so basically sqroot ( (-x)^2) = -x..... similarly sqroot (-a) * sqroot(-b) is not equal to sqroot(ab)... but is equal to - sqroot(ab)... hope this helps...

Send private message

momentary_lapse
Rising GMAT Star

Joined: 26 Feb 2007
Posts: 43

Thanks given: 0
Thanked 0 times in 0 posts

Tue Mar 27, 2007 12:00 pm

Well the questions says square root of (-x\|x\|) where x<0 So it seems to be asking for the square root of the product of -x and \|x\| Im not sure how square root of (-x)(-x) is -x Which property of complex numbers is this?

Send private message

aim-wsc
GMAT Titan

Joined: 20 Apr 2006
Posts: 2469

Thanks given: 144
Thanked 57 times in 45 posts
Location: BtG Underground

Target GMAT Score: 801-

Tue Mar 27, 2007 12:37 pm

gabriel wrote: Ok.. the q asks to find the sq root ( -x mod (x))= sq root ( -x )* sq root ((mod(x)) = sq root (-x)* sq root(-x)... let this be eqn no. 1.... .. for understanding this u need to know the basics of complex numbers... the root of a negative qty -x can be written as sqroot (x)* sqroot (-1)... so applying this to eqn 1.... we get sqroot (x)sqroot(-1) sqroot(x)sqroot(-1) .... that is sqroot (x)sqroot(x)sqroot(-1)sqroot(-1)... which is equal to -x... so basically sqroot ( (-x)^2) = -x..... similarly sqroot (-a) * sqroot(-b) is not equal to sqroot(ab)... but is equal to - sqroot(ab)... hope this helps... wow! but dont make it complex by bringing complex numbers here! even by hearing the word I almost got an heart attack there OK let me try 8) Quote: 1) If x<0 , what is square root of -x\|x\| I thought if x<0, \|x\| = -x THerefore square root becomes the root of x^2 which is + or - x I wrote the answer as x but the GMAT Prep says that answer is -x. How??? first off: \|x\| CANNOT BE equal to -x (never) now -x\|x\| = (-1)* (x) * mod (x).............(remember x has -ve value already! since x<0) so let x= (-1)* x' where x'= (+ve) x =(-1)* (-1)x' * x' = (-1)^2 * x'^2 I think it is very easy to sq rooting above value. which equals to =(-1)* x' =-x _________________ Getting started @BTG? Beginner's Guide to GMAT \| Johny learns to blog Please do not PM me, (not active anymore) contact Eric.

Send private message

Visit poster's website

Display posts from previous:

	All times are GMT - 7 Hours Goto page 1, 2 Next
Page 1 of 2

Forum Index GMAT Math

Most Active Members in Last 30 Days

1.

kstv

322 posts

2.

shashank.ism

312 posts

3.

harsh.champ

308 posts

4.

gmatmachoman

256 posts

5.

thephoenix

238 posts

See More Top Beat The GMAT Members

Most Active Experts in Last 30 Days

1.

lunarpower
Manhattan GMAT Teacher

97 posts

2.

Stuart Kovinsky
Kaplan GMAT Teacher

58 posts

3.

Testluv
Kaplan GMAT Teacher

51 posts

4.

Lisa Anderson
Stacy Blackman Consulting

49 posts

5.

Bryant@VeritasPrep
Veritas Prep

41 posts

See More Top Beat The GMAT Experts