If k is an integer and x(x - k) = k + 1, what is the value of x?
(1) x < k
(2) x = 3 - k
Source: GMATPrepnow
OA: A
Tricky DS Question
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- OptimusPrep
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Given: x(x-k) = k + 1Mo2men wrote:If k is an integer and x(x - k) = k + 1, what is the value of x?
(1) x < k
(2) x = 3 - k
x^2 - xk -k - 1 = 0
x^2 - 1 -k(x+1) = 0
(x-1)(x+1) - k (x+1) = 0
(x+1)(x-1-k) = 0 - (i)
Hence x = - 1 or x = 1+k
Required: x = ?
Statement 1: x < k
From this, we can say that ≠k+1
Hence x can only take the value -1
SUFFICIENT
Statement 2: x = 3-k
On substituting the value of k in the solutions for the equation, we get
3-k = - 1 and 3-k = 1+k
Hence k = 4 and k = 1
INSUFFICIENT
Correct Option: A
A very good question which can trap you in choosing the option C
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Thanks Ankur for you help.OptimusPrep wrote:Given: x(x-k) = k + 1Mo2men wrote:If k is an integer and x(x - k) = k + 1, what is the value of x?
(1) x < k
(2) x = 3 - k
x^2 - xk -k - 1 = 0
x^2 - 1 -k(x+1) = 0
(x-1)(x+1) - k (x+1) = 0
(x+1)(x-1-k) = 0 - (i)
Hence x = - 1 or x = 1+k
Required: x = ?
Statement 1: x < k
From this, we can say that ≠k+1
Hence x can only take the value -1
SUFFICIENT
Statement 2: x = 3-k
On substituting the value of k in the solutions for the equation, we get
3-k = - 1 and 3-k = 1+k
Hence k = 4 and k = 1
INSUFFICIENT
Correct Option: A
A very good question which can trap you in choosing the option C
I have a question regarding statement 1. I do not understand how it satisfies the condition that x<k when x=-1. We do not know what the value of k except that we substitute x=-1 in equation and get k=0 then K>x.
Am I right?
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Statement 1: x<kMo2men wrote:If k is an integer and x(x - k) = k + 1, what is the value of x?
(1) x < k
(2) x = 3 - k
Case 1: k=0
Substituting k=0 into x(x - k) = k + 1, we get:
x(x - 0) = 0 + 1
x² = 1
x = ±1.
Since statement 1 requires that x<k, the only viable option is x=-1.
Case 2: k=10
Substituting k=10 into x(x - k) = k + 1, we get:
x(x - 10) = 10 + 1
x² - 10x = 11
x² - 10x - 11 = 0
(x-11)(x+1) = 0
x = 11 or x=-1.
Since statement 1 requires that x<k, the only viable option is x=-1.
Case 3: k=-1
Substituting k=-1 into x(x - k) = k + 1, we get:
x(x + 1) = -1 + 1
x² + x = 0
x = 0.
Since statement 1 requires that x<k, this case is not viable.
Case 4: k=-10
Substituting k=-10 into x(x - k) = k + 1, we get:
x(x + 10) = -10 + 1
x² + 10x = -9
x² + 10x + 9 = 0
(x+9)(x+1) = 0
x = -9 or x=-1.
Since neither option for x is such that x<k, this case is not viable.
In every viable case, x=-1.
SUFFICIENT.
Statement 2: x=3-k
Substituting k = 3-x into x(x - k) = k + 1, we get:
x(x - (3-x)) = 3-x + 1
x(2x - 3) = 4 - x
2x² - 3x = 4 - x
2x² - 2x - 4 = 0
x² - x - 2 = 0
(x-2)(x+1) = 0
x=2 or x=-1.
Since x can be different values, INSUFFICIENT.
The correct answer is A.
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- Brent@GMATPrepNow
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This is a challenge question I created for BTG a while back.Mo2men wrote:If k is an integer and x(x - k) = k + 1, what is the value of x?
(1) x < k
(2) x = 3 - k
Here's a video solution - https://www.beatthegmat.com/mba/2010/09/ ... er-13-2010
Cheers,
Brent
Last edited by Brent@GMATPrepNow on Tue Jul 19, 2016 5:45 am, edited 1 time in total.
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Hi Brent,Brent@GMATPrepNow wrote:This is a challenge question I created for BTG a while back.Mo2men wrote:If k is an integer and x(x - k) = k + 1, what is the value of x?
(1) x < k
(2) x = 3 - k
Here's a video solution.
Cheers,
Brent
There is not video attached.
Thanks
GMAT/MBA Expert
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Ooops.Mo2men wrote:
Hi Brent,
There is not video attached.
Thanks
It is now.
Cheers,
Brent
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If we haveMo2men wrote:I have a question regarding statement 1. I do not understand how it satisfies the condition that x<k when x=-1. We do not know what the value of k except that we substitute x=-1 in equation and get k=0 then K>x.
Am I right?
x = k + 1 or x = -1
we know that the first of these (x = k + 1) gives us
x = k + 1
k + 1 > k
so
x > k
S1 tells us that x is NOT greater than k, so x = k + 1 must NOT be the root that we're looking for. That means we want the other root: x = -1.