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What is the value of y?

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Manager
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What is the value of y? [#permalink] New post 04 Dec 2012, 22:23
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  5% (low)

Question Stats:

52% (01:43) correct 47% (01:04) wrong based on 21 sessions
What is the value of y?

(1) y^3+2y=y+2y^2

(2) y^2=y
[Reveal] Spoiler: OA

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Manager
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Joined: 04 Oct 2011
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Location: India
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GMAT 1: 440 Q33 V13
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Kudos [?]: 24 [0], given: 44

Re: What is the value of y? [#permalink] New post 04 Dec 2012, 22:30
shanmugamgsn wrote:
What is the value of y?

(1)y^3+2y=y+2y^2

(2)y^2=y



My choice was A..
I did this way can any one correct me if im wrong??

(2) INSUFFICIENT :
y^2-y=0
y(y-1)
So y=0 or y=1

(1) i felt this is SUFFICIENT:
y^3+2y=y+2y^2
divide by y on both sides
y^2+2=1+2y
y^2-2y+1=0
(y-1)^2=0
So solution will be y=1

Is there any mistakes in my calculation?
If y this soln is wrong???
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Re: What is the value of y? [#permalink] New post 04 Dec 2012, 23:42
shanmugamgsn wrote:
shanmugamgsn wrote:
What is the value of y?

(1)y^3+2y=y+2y^2

(2)y^2=y



My choice was A..
I did this way can any one correct me if im wrong??

(2) INSUFFICIENT :
y^2-y=0
y(y-1)
So y=0 or y=1

(1) i felt this is SUFFICIENT:
y^3+2y=y+2y^2
divide by y on both sides
y^2+2=1+2y
y^2-2y+1=0
(y-1)^2=0
So solution will be y=1

Is there any mistakes in my calculation?
If y this soln is wrong???


At first glance of the equation given in Statement 1, you should be able to see that 0 is a solution to the equation. Every expression contains a y term and hence 0 will satisfy the equation. Your first step is dividing by y. Since y is an unknown variable, that step cannot be allowed. As the famous mems of the internet go No one except Chuck Norris can divide by 0. :P...
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Re: What is the value of y? [#permalink] New post 05 Dec 2012, 01:02
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Re: What is the value of y? [#permalink] New post 05 Dec 2012, 01:04
Expert's post
shanmugamgsn wrote:
shanmugamgsn wrote:
What is the value of y?

(1)y^3+2y=y+2y^2

(2)y^2=y



My choice was A..
I did this way can any one correct me if im wrong??

(2) INSUFFICIENT :
y^2-y=0
y(y-1)
So y=0 or y=1

(1) i felt this is SUFFICIENT:
y^3+2y=y+2y^2
divide by y on both sides
y^2+2=1+2y
y^2-2y+1=0
(y-1)^2=0
So solution will be y=1

Is there any mistakes in my calculation?
If y this soln is wrong???


Never reduce an equation by a variable (or expression with a variable), if you are not certain that the variable (or the expression with a variable) doesn't equal to zero. We can not divide by zero.

So, if you divide (reduce) y^3+2y=y+2y^2 by y, you assume, with no ground for it, that y does not equal to zero thus exclude a possible solution (notice that both y=1 AND y=0 satisfy the equation).

Hope it's clear.
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Manager
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Joined: 04 Oct 2011
Posts: 227
Location: India
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Kudos [?]: 24 [0], given: 44

Re: What is the value of y? [#permalink] New post 05 Dec 2012, 02:44
MacFauz wrote:
shanmugamgsn wrote:
shanmugamgsn wrote:
What is the value of y?

(1)y^3+2y=y+2y^2

(2)y^2=y



My choice was A..
I did this way can any one correct me if im wrong??

(2) INSUFFICIENT :
y^2-y=0
y(y-1)
So y=0 or y=1

(1) i felt this is SUFFICIENT:
y^3+2y=y+2y^2
divide by y on both sides
y^2+2=1+2y
y^2-2y+1=0
(y-1)^2=0
So solution will be y=1

Is there any mistakes in my calculation?
If y this soln is wrong???


At first glance of the equation given in Statement 1, you should be able to see that 0 is a solution to the equation. Every expression contains a y term and hence 0 will satisfy the equation. Your first step is dividing by y. Since y is an unknown variable, that step cannot be allowed. As the famous mems of the internet go No one except Chuck Norris can divide by 0. :P...



Thanks MacFauz....

Actually, i forgot this rule :oops:
_________________

GMAT - Practice, Patience, Persistence
Kudos if u like :)

Manager
Manager
User avatar
Joined: 04 Oct 2011
Posts: 227
Location: India
Concentration: Entrepreneurship, International Business
GMAT 1: 440 Q33 V13
GMAT 2: 0 Q0 V0
GPA: 3
Followers: 0

Kudos [?]: 24 [0], given: 44

Re: What is the value of y? [#permalink] New post 05 Dec 2012, 02:45
Bunuel wrote:
shanmugamgsn wrote:
shanmugamgsn wrote:
What is the value of y?

(1)y^3+2y=y+2y^2

(2)y^2=y



My choice was A..
I did this way can any one correct me if im wrong??

(2) INSUFFICIENT :
y^2-y=0
y(y-1)
So y=0 or y=1

(1) i felt this is SUFFICIENT:
y^3+2y=y+2y^2
divide by y on both sides
y^2+2=1+2y
y^2-2y+1=0
(y-1)^2=0
So solution will be y=1

Is there any mistakes in my calculation?
If y this soln is wrong???


Never reduce an equation by a variable (or expression with a variable), if you are not certain that the variable (or the expression with a variable) doesn't equal to zero. We can not divide by zero.

So, if you divide (reduce) y^3+2y=y+2y^2 by y, you assume, with no ground for it, that y does not equal to zero thus exclude a possible solution (notice that both y=1 AND y=0 satisfy the equation).

Hope it's clear.



Thanks Bunuel....
made me clear...
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Kudos if u like :)

Re: What is the value of y?   [#permalink] 05 Dec 2012, 02:45
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