Last visit was: 23 Nov 2024, 15:12 It is currently 23 Nov 2024, 15:12

Close

GRE Prep 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 GRE 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.

Close

Request Expert Reply

Confirm Cancel
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Own Kudos [?]: 12196 [16]
Given Kudos: 136
Send PM
Most Helpful Community Reply
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Own Kudos [?]: 12196 [5]
Given Kudos: 136
Send PM
General Discussion
avatar
Retired Moderator
Joined: 16 Oct 2019
Posts: 63
Own Kudos [?]: 175 [1]
Given Kudos: 21
Send PM
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Own Kudos [?]: 12196 [0]
Given Kudos: 136
Send PM
Re: The equation x + y = xy = x/y has how many [#permalink]
2
vndnjn wrote:
why x = 0 and y = 0, not a solution??


Great question!

If x = 0 and y = 0, then x + y = 0, and xy = 0, but x/y does not equal 0.
The fraction 0/0 is undefined (just like the fraction 5/0 is undefined)
That is, 0/0 does not have any real numerical value.
As such. we can't say that 0/0 = 0

Cheers,
Brent
avatar
Manager
Manager
Joined: 22 Jan 2020
Posts: 120
Own Kudos [?]: 240 [2]
Given Kudos: 10
Send PM
Re: The equation x + y = xy = x/y has how many [#permalink]
2
x+y=xy=(x/y)

we break this into three separate equalities

(1) x+y=xy
(2) xy=(x/y)
(3) x+y=x/y

Remember we need solutions for that work for all (1),(2), and (3)

From (2) we get
xy^2=x
xy^2-x=0
x(y^2-1)=0
x=0, y=1, y=-1

From (1) we get

if x=0
y=0, but y can't be zero or else (x/y) would be undefined

if y=1
x+1=x
1=0, this is a contradiction, therefore y is not 1

if y=-1
x-1=-x
2x=1
x=1/2

So we have the solution x=1/2, y=-1

Let's see if it finally works in (3)


(1/2)*-1=(1/2)/(-1)
-1/2=-1/2
This is true.


Therefore we have that x=1/2 and y=-1 is the only solution that works

Final Answer: B
Intern
Intern
Joined: 02 Mar 2022
Posts: 17
Own Kudos [?]: 5 [0]
Given Kudos: 37
Send PM
Re: The equation x + y = xy = x/y has how many [#permalink]
Thank you for explaining! My only question is, why is the answer only ONE solution. Wouldn't it be two solutions, x = 0.5, and y = -1? THANK YOU! Or are negatives not considered "real" solutions?


GreenlightTestPrep wrote:
GreenlightTestPrep wrote:
The equation \(x + y = xy = \frac{x}{y}\) has how many real solutions?

A) 0
B) 1
C) 2
D) 3
E) More than 3


To begin, if \(xy = \frac{x}{y}\), we can conclude that \(y\neq0\), otherwise \(\frac{x}{y}\) is undefined.

Take: \(xy = \frac{x}{y}\)
Multiply both sides of the equation by \(y\) to get: \(xy^2 = x\)
Subtract \(x\) from both sides to get: \(xy^2 - x = 0\)
Factor to get: \(x(y^2 - 1) = 0\)
Factor again to get: \(x(y - 1)(y + 1) = 0\)
There are 3 POSSIBLE solutions to the above equation: \(x = 0\), \(y = 1\) and \(y = -1\)

Let's examine each possible case:

case i: \(x = 0\)
Substitute this into the original equation to get: \(0 + y = (0)y = \frac{0}{y}\)
For this equation to hold true, we need \(y = 0\), but we already showed that \(y\) cannot equal \(0\)
So, it can't be the case that \(x = 0\)

case ii: \(y = 1\)
Substitute this into the original equation to get: \(x + 1 = x(1) = \frac{x}{1}\)
Simplify: \(x + 1 = x = x\)
Since there are no solutions to the equation \(x + 1 = x\), it can't be the case that \(y = 1\)

case iii: \(y = -1\)
Substitute this into the original equation to get: \(x + (-1) = x(-1) = \frac{x}{-1}\)
Simplify: \(x - 1 = -x = -x\)
Now take: \(x - 1 = -x\)
Add \(x\) to both sides: \(2x - 1 = 0\)
Add \(1\) to both sides: \(2x = 1\)
Solve: \(x = 0.5\)
So, \(x = 0.5\) and \(y = -1\) is the ONLY possible solution to the given equation.

Answer: B

Cheers,
Brent
Verbal Expert
Joined: 18 Apr 2015
Posts: 30016
Own Kudos [?]: 36366 [1]
Given Kudos: 25928
Send PM
The equation x + y = xy = x/y has how many [#permalink]
1
Expert Reply
0 makes the fraction UNDEFINED so it is not possible

1 makes the equation x+1=x

x-x+1=0

0+1=0

1=0 so it is not possible

-1 is our value

if you insert -1 and obtain a cogent value (0.5) so our equation has ONE solution valid
Intern
Intern
Joined: 05 Jan 2023
Posts: 22
Own Kudos [?]: 16 [0]
Given Kudos: 35
GRE 1: Q164 V160
Send PM
Re: The equation x + y = xy = x/y has how many [#permalink]
Thanks to you all for the solution.

What concept(s) does this question test?

I can see divisibility/multiples/fractions but it seems to be testing systems of equations, silmultaneous equations etc.

Please help.
Verbal Expert
Joined: 18 Apr 2015
Posts: 30016
Own Kudos [?]: 36366 [1]
Given Kudos: 25928
Send PM
The equation x + y = xy = x/y has how many [#permalink]
1
Expert Reply
Intern
Intern
Joined: 05 Jan 2023
Posts: 22
Own Kudos [?]: 16 [0]
Given Kudos: 35
GRE 1: Q164 V160
Send PM
Re: The equation x + y = xy = x/y has how many [#permalink]
Thanks so much!

I will look at them.
User avatar
GRE Prep Club Legend
GRE Prep Club Legend
Joined: 07 Jan 2021
Posts: 5043
Own Kudos [?]: 74 [0]
Given Kudos: 0
Send PM
Re: The equation x + y = xy = x/y has how many [#permalink]
Hello from the GRE Prep Club BumpBot!

Thanks to another GRE Prep 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.
Prep Club for GRE Bot
Re: The equation x + y = xy = x/y has how many [#permalink]
Moderators:
GRE Instructor
84 posts
GRE Forum Moderator
37 posts
Moderator
1111 posts
GRE Instructor
234 posts

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne