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If x is a negative integer, what is the value of x?

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If x is a negative integer, what is the value of x?  [#permalink]

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New post 24 Nov 2019, 12:14
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If \(x\) is a negative integer, what is the value of \(x\)?

(1) \(x^2 + 40x + 398 < 0\)

(2) \((x+17)(x+20) < 0\)

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If x is a negative integer, what is the value of x?  [#permalink]

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New post 24 Nov 2019, 19:59
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If \(x\) is a negative integer, what is the value of \(x\)?

(1) \(x^2 + 40x + 398 < 0......x(x+40)+398<0\)
x(x+40)<-398
This will happen when x is between 0 and -40..
x =-20......x(x+40)=-20*20=-400
x=-19.......-19*21=-399
Insuff

(2) \((x+17)(x+20) < 0\)
This means x lies between-17 and -20, so x can be -18 or -19.
Insuff

Combined
x =-19
Suff

C
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Re: If x is a negative integer, what is the value of x?  [#permalink]

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New post 25 Nov 2019, 10:51
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GMATPrepNow wrote:
If \(x\) is a negative integer, what is the value of \(x\)?

(1) \(x^2 + 40x + 398 < 0\)

(2) \((x+17)(x+20) < 0\)

Given: x is a negative integer

Target question: What is the value of x?

Statement 1: \(x^2 + 40x + 398 < 0\)
If we recognize that x² + 40x + 398 is very close to the PERFECT SQUARE x² + 40x + 400, we can quickly deal with statement 1
Take: x² + 40x + 398 < 0
Add 2 to both sides to get: x² + 40x + 400 < 2
Factor: (x + 20)(x + 20) < 2
In other words: (x + 20)² < 2
Since we're told x is an INTEGER, we can see that there are 3 values of x that satisfy the inequality (x + 20)² < 2
So, x can equal -21, -20 or -19

Statement 2: \((x+17)(x+20) < 0\)
Notice that, when -20 < x < -17, we see that (x+17)(x+20) = (NEGATIVE)(POSITIVE) = NEGATIVE
In other words, when x is BETWEEN -20 and -17, (x+17)(x+20) < 0
So, x can equal -18 or -19
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Statement 1 tells us that x can equal -21, -20 or -19
Statement 2 tells us that x can equal -18 or -19
Since both statements are TRUE, it must be the case that x = -19
Since we can answer the target question with certainty, the combined statements are SUFFICIENT

Answer: C

Cheers,
Brent
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Re: If x is a negative integer, what is the value of x?  [#permalink]

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New post 25 Nov 2019, 11:59
GMATPrepNow How did you get from: (x+17)(x+20)<0(x+17)(x+20)<0 to -20 < x < -17? My first instinct was to get x<-17 and x<-20?
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Re: If x is a negative integer, what is the value of x?  [#permalink]

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New post 25 Nov 2019, 12:06
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firsttimenoob wrote:
GMATPrepNow How did you get from: (x+17)(x+20)<0(x+17)(x+20)<0 to -20 < x < -17? My first instinct was to get x<-17 and x<-20?


First notice that (x+17)(x+20) = 0, when x = -17 and when x = -20
Let's examine 3 possible ranges for the value of x.

case i x < -20
If x < -20, then (x+17) is NEGATIVE, and (x+20) is NEGATIVE
So, (x+17)(x+20) = (NEGATIVE)(NEGATIVE) = POSITIVE
So, when x < -20, x is NOT a solution to the inequality (x+17)(x+20) < 0

case ii -20 < x < -17
If -20 < x < -17 then (x+17) is NEGATIVE, and (x+20) is POSITIVE
So, (x+17)(x+20) = (NEGATIVE)(POSITIVE) = NEGATIVE
So, when -20 < x < -17, x IS a solution to the inequality (x+17)(x+20) < 0

case iii x > -17
If x > -17, then (x+17) is POSITIVE, and (x+20) is POSITIVE
So, if (x+17)(x+20) = (POSITIVE)(POSITIVE) = POSITIVE
So, when x > -17, x is NOT a solution to the inequality (x+17)(x+20) < 0

Does that help?

Cheers,
Brent
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Re: If x is a negative integer, what is the value of x?   [#permalink] 25 Nov 2019, 12:06
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