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11 Feb 2017, 07:39
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B
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Difficulty:
75%
(hard)
Question Stats:
52% (02:14) correct
48%
(02:27)
wrong
based on 388
sessions
History
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If x is an integer, what is the value of x?
(1) x2 + 8x + 16 > 1
(2) x2 – 8x + 9 < -6
* Kudos for all correct solutions
(1) x2 + 8x + 16 > 1
(2) x2 – 8x + 9 < -6
* Kudos for all correct solutions
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11 Feb 2017, 08:06
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(1) x² + 8x + 15 > 0
or (x+3)(x+5)>0
thus x<-5 or x>-3
insuff
(2)x² – 8x + 15 < 0
(x-5)(x-3)<0
thus 3<x<5
thus x=4
suff
Ans B
12 Feb 2017, 10:36
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GMATPrepNow
Target question: What is the value of x?
Statement 1: x² + 8x + 16 > 1
Subtract 1 from both sides to get: x² + 8x + 15 > 0
We might see right away that MANY different values of x will satisfy this inequality.
To begin, any POSITIVE value of x will satisfy the inequality x² + 8x + 15 > 0
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: x² – 8x + 9 < -6
Add 6 to both sides to get: x² - 8x + 15 < 0
Factor to get: (x - 3)(x - 5) < 0
With quadratic inequalities like this, it's useful to first examine and solve the corresponding EQUATION
So, solve: (x - 3)(x - 5) = 0
We get: x = 3 or x = 5
These are our CRITICAL VALUES of x (i.e., x-values that satisfy the corresponding EQUATION)
Now let's examine what happens within the x-values LESS THAN and GREATER THAN these critical values.
There are 3 ranges to consider:
i) x is less than 3
ii) x is between 3 and 5
iii) x is greater than 5
i) x is less than 3
If x < 3, then (x - 3) is NEGATIVE and (x - 5) is NEGATIVE
So, (x - 3)(x - 5) = (NEGATIVE)(NEGATIVE) = POSITIVE
So, when x is less than 3, (x - 3)(x - 5) > 0
We're looking for x-values that satisfy the inequality (x - 3)(x - 5) < 0, so we can conclude that x is NOT less than 3
ii) x is between 3 and 5
If 3 < x < 5, then (x - 3) is POSTIVE and (x - 5) is NEGATIVE
So, (x + 3)(x + 5) = (POSITIVE)(NEGATIVE) = NEGATIVE
So, when x is between 3 and 5, (x - 3)(x - 5) < 0
Perfect, we're looking for x-values that satisfy the inequality (x - 3)(x - 5) < 0!
So we can conclude that x IS between 3 and 5
iii) x is greater than 5
If x > 5, then (x - 3) is POSITIVE and (x - 5) is POSITIVE
So, (x - 3)(x - 5) = (POSITIVE)(POSITIVE) = POSITIVE
So, x is greater than 5, (x - 3)(x - 5) > 0
We're looking for x-values that satisfy the inequality (x - 3)(x - 5) < 0, so we can conclude that x is NOT greater than 5
We have seen that x IS between 3 and 5
Since 4 is the ONLY integer between 3 and 5, we can be certain that x = 4
Since we can answer the target question with certainty, statement 2 is SUFFICIENT
Answer: B
Cheers,
Brent
12 Feb 2017, 15:04
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My solution based on visual representation
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