NEW HERE? LEARN MORE
closeclose
Last visit was: 26 Apr 2024, 11:12 It is currently 26 Apr 2024, 11:12
Decision Tracker
My Rewards
New posts
New comers' posts

Close
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
Your Progress

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
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel

If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu

SORT BY:
Date
Kudos
Tags:
Find Similar Topics 
Show Tags
Hide Tags
User avatar
L
Bunuel
Math Expert
Joined: 02 Sep 2009
Posts: 92948
Own Kudos [?]: 619215 [9]
Given Kudos: 81609
Send PM
CAT Tests Forum Quiz Mode
If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   05 Jul 2018, 21:28
8
Bookmarks
Expert Reply
00:00
A
B
C
D
E

Difficulty:

35% (medium)

Question Stats:

71% (01:45) correct 29% (02:05) wrong based on 213 sessions
Hide Show timer Statistics
If \(y^{-2} + 2y^{-1} -15 = 0\), which of the following could be the value of y?


A. \(3\)

B. \(\frac{1}{5}\)

C. \(\frac{-1}{5}\)

D. \(\frac{-1}{3}\)

E. \(-5\)
ShowHide Answer
Official Answer
C
User avatar
D
PKN
Director
Director
Joined: 01 Oct 2017
Status:Learning stage
Posts: 827
Own Kudos [?]: 1300 [3]
Given Kudos: 41
WE:Supply Chain Management (Energy and Utilities)
Send PM
If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   05 Jul 2018, 21:38
3
Kudos
Bunuel wrote:
If \(y^{-2} + 2y^{-1} -15 = 0\), which of the following could be the value of y?


A. \(3\)

B. \(\frac{1}{5}\)

C. \(\frac{-1}{5}\)

D. \(\frac{-1}{3}\)

E. \(-5\)


Let \(y^{-1}=x\)

So, \(y^{-2} + 2y^{-1} -15 = 0\)
Or, \(x^2+2x-15=0\)
Or, x=-5 or x=3

1. x=-5, or, \(y^{-1}=-5\) or, \(y=\frac{-1}{5}\)
2. x=3,or, \(y^{-1}=3\) or, \(y=\frac{1}{3}\)

Among given options, correct answer is (C)
User avatar
D
PKN
Director
Director
Joined: 01 Oct 2017
Status:Learning stage
Posts: 827
Own Kudos [?]: 1300 [0]
Given Kudos: 41
WE:Supply Chain Management (Energy and Utilities)
Send PM
Re: If y^(-2) + 2y^(-1) -15 = 0, which of the following could be the value [#permalink] New post   08 Jul 2018, 05:26
Bunuel wrote:
If \(y^{-2} + 2y^{-1} -15 = 0\), which of the following could be the value of y?


A. \(3\)

B. \(\frac{1}{5}\)

C \(\frac{-1}{5}\)

D. \(\frac{-1}{3}\)

E. \(-5\)


Let \(y^{-1}=x\)

So, \(y^{-2} + 2y^{-1} -15 = 0\)
Or, \(x^2+2x-15=0\)
Or, x=-5 or x=3

1. x=-5, or, \(y^{-1}=-5\) or, \(y=\frac{-1}{5}\)
2. x=3,or, \(y^{-1}=3\) or, \(y=\frac{1}{3}\)

Among given options, correct answer is (C)[/quote]
P.S:- Already discussed .
https://gmatclub.com/forum/if-y-2-2y-1- ... 69804.html
User avatar
B
Seryozha
Intern
Intern
Joined: 04 Aug 2017
Status:No Progress without Struggle
Posts: 40
Own Kudos [?]: 17 [0]
Given Kudos: 108
Location: Armenia
GPA: 3.4
Send PM
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   09 Jul 2018, 03:44
PKN wrote:
Bunuel wrote:
If \(y^{-2} + 2y^{-1} -15 = 0\), which of the following could be the value of y?


A. \(3\)

B. \(\frac{1}{5}\)

C. \(\frac{-1}{5}\)

D. \(\frac{-1}{3}\)

E. \(-5\)


Let \(y^{-1}=x\)

So, \(y^{-2} + 2y^{-1} -15 = 0\)
Or, \(x^2+2x-15=0\)
Or, x=-5 or x=3

1. x=-5, or, \(y^{-1}=-5\) or, \(y=\frac{-1}{5}\)
2. x=3,or, \(y^{-1}=3\) or, \(y=\frac{1}{3}\)

Among given options, correct answer is (C)

Is there an alternative way to solve this problem?
User avatar
D
PKN
Director
Director
Joined: 01 Oct 2017
Status:Learning stage
Posts: 827
Own Kudos [?]: 1300 [0]
Given Kudos: 41
WE:Supply Chain Management (Energy and Utilities)
Send PM
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   09 Jul 2018, 03:51
Plugin answer choices and put in LHS of the given equation.The value of y for which the LHS becomes zero is the correct answer choice.

Posted from my mobile device
User avatar
L
Chethan92
Director
Director
Joined: 18 Jul 2018
Posts: 926
Own Kudos [?]: 1288 [1]
Given Kudos: 95
Location: India
Concentration: Operations, General Management
Schools: IIMB EPGP'22 (M)
GMAT 1: 590 Q46 V25
GMAT 2: 690 Q49 V34
WE:Engineering (Energy and Utilities)
Send PM
Reviews Badge
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   22 Aug 2018, 06:54
1
Kudos
\(y^{-2}\) can be written as \(\frac{1}{y^2}\)
Similarly \(y^{-1}\) can be written as \(\frac{1}{y}\)

Then the equation becomes \(\frac{1}{y^2}\) + 2\(\frac{1}{y}\) - 15 = 0.

Taking LCM, the equation becomes 1+2y-15\(y^2\) = 0.

Re-arranging gives 15\(y^2\)-2y-1 = 0.

15\(y^2\)-5y+3y-1 = 0.

5y(3y-1)+1(3y-1) = 0

Either y = \(\frac{-1}{3}\) or y = \(\frac{-1}{5}\).

Among the answer choices.

y = \(\frac{-1}{5}\)

C is the answer.
User avatar
P
bebs
Senior Manager
Senior Manager
Joined: 18 Jun 2018
Posts: 334
Own Kudos [?]: 199 [0]
Given Kudos: 1283
Concentration: Finance, Healthcare
Schools: Jones '22 (M$) McCombs '22 (WL) Kelley '22 (A$)
Send PM
If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   22 Aug 2018, 08:27
Afc0892

Sure it is a typo (since you got the answer) but -1/3 is not a root, otherwise option D will be correct too. The roots are 1/3 and -1/5 (option C). I am also an arsenal fan. Up gunners!!!


Afc0892 wrote:
\(y^{-2}\) can be written as \(\frac{1}{y^2}\)
Similarly \(y^{-1}\) can be written as \(\frac{1}{y}\)

Then the equation becomes \(\frac{1}{y^2}\) + 2\(\frac{1}{y}\) - 15 = 0.

Taking LCM, the equation becomes 1+2y-15\(y^2\) = 0.

Re-arranging gives 15\(y^2\)-2y-1 = 0.

15\(y^2\)-5y+3y-1 = 0.

5y(3y-1)+1(3y-1) = 0

Either y = \(\frac{-1}{3}\) or y = \(\frac{-1}{5}\).

Among the answer choices.

y = \(\frac{-1}{5}\)

C is the answer.
avatar
B
mandy0rhtdm
Intern
Intern
Joined: 14 Sep 2014
Posts: 13
Own Kudos [?]: 4 [0]
Given Kudos: 30
Location: India
Send PM
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   22 Aug 2018, 08:32
The best way to go here is to put the options.
And only select options that is any option more than 1 will not satisfy.
Only options less than 1 will satisfy since the power is negative.

Posted from my mobile device
User avatar
L
Chethan92
Director
Director
Joined: 18 Jul 2018
Posts: 926
Own Kudos [?]: 1288 [0]
Given Kudos: 95
Location: India
Concentration: Operations, General Management
Schools: IIMB EPGP'22 (M)
GMAT 1: 590 Q46 V25
GMAT 2: 690 Q49 V34
WE:Engineering (Energy and Utilities)
Send PM
Reviews Badge
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   22 Aug 2018, 17:55
funsogu wrote:
Afc0892

Sure it is a typo (since you got the answer) but -1/3 is not a root, otherwise option D will be correct too. The roots are 1/3 and -1/5 (option C). I am also an arsenal fan. Up gunners!!!


Afc0892 wrote:
\(y^{-2}\) can be written as \(\frac{1}{y^2}\)
Similarly \(y^{-1}\) can be written as \(\frac{1}{y}\)

Then the equation becomes \(\frac{1}{y^2}\) + 2\(\frac{1}{y}\) - 15 = 0.

Taking LCM, the equation becomes 1+2y-15\(y^2\) = 0.

Re-arranging gives 15\(y^2\)-2y-1 = 0.

15\(y^2\)-5y+3y-1 = 0.

5y(3y-1)+1(3y-1) = 0

Either y = \(\frac{-1}{3}\) or y = \(\frac{-1}{5}\).

Among the answer choices.

y = \(\frac{-1}{5}\)

C is the answer.


Yes its typo. Thanks for mentioning. COYG ?

Posted from my mobile device
User avatar
L
ScottTargetTestPrep
Target Test Prep Representative
Joined: 14 Oct 2015
Status:Founder & CEO
Affiliations: Target Test Prep
Posts: 18766
Own Kudos [?]: 22061 [0]
Given Kudos: 283
Location: United States (CA)
Send PM
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   26 Aug 2018, 18:21
Expert Reply
Bunuel wrote:
If \(y^{-2} + 2y^{-1} -15 = 0\), which of the following could be the value of y?


A. \(3\)

B. \(\frac{1}{5}\)

C. \(\frac{-1}{5}\)

D. \(\frac{-1}{3}\)

E. \(-5\)


Simplifying we have:

1/y^2 + 2/y - 15 = 0

Multiplying by y^2 we have:

1 + 2y - 15y^2 = 0

15y^2 - 2y - 1 = 0

(3y - 1)(5y + 1) = 0

3y - 1 = 0 → 3y = 1 → y = 1/3

Or

5y + 1 = 0 → 5y = -1 → y = -⅕

Answer: C
User avatar
bumpbot
Non-Human User
Joined: 09 Sep 2013
Posts: 32688
Own Kudos [?]: 822 [0]
Given Kudos: 0
Send PM
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink] New post   15 Dec 2022, 09:28
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
GMAT Club Bot
gmatclubot
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu [#permalink]
15 Dec 2022, 09:28
Moderators:
Bunuel
Math Expert
92947 posts
yashikaaggarwal
Senior Moderator - Masters Forum
3137 posts

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne
Main navigation
GMAT Resources
Partners
MBA Resources
www.gmac.com|www.mba.com | | GMAT Club Rules | Terms and Conditions