DevB wrote:Is √[(x - 3)²] = 3 - x ?
1. x is not equal to 3
2. -x|x| > 0
Target question:
Is √[(x - 3)²] = 3 - x ?
This question is a great candidate for rephrasing the target question.
Aside: We have a free video with tips on rephrasing the target question: https://www.gmatprepnow.com/module/gmat- ... cy?id=1100
To begin, notice that we have two nice rules:
- If k > 0, then √(k²) = k
- If k < 0, then √(k²) = -k
Now observe that (3-x) = -(x-3)
Given the above information, under what conditions will
√[(x-3)²] = 3-x?
In other words, under what conditions will √[(x-3)²] = -(x-3)?
This will occur IF x-3 is NEGATIVE.
So, we can now rephrase the target question as:
Is (x-3) negative?
Or we can write:
Is x-3 < 0?
. . . or better yet:
Is x < 3?
Now that we've REPHRASED the target question in much simpler terms, we can check the statements.
Statement 1: x not equal to 3
This does not give us a definitive answer to the REPHRASED target question (
Is x < 3? )
As such, statement 1 is NOT SUFFICIENT
Statement 2: -x|x| > 0
First notice that this implies that x does not equal zero.
Next, notice that, if x does not equal zero, then |x| will always be
positive.
So, -x|x| > 0 is the same as saying (-x)(
positive) > 0
In other words, the product (-x)(
positive) results in a positive number.
This tells us that (-x) must be positive, which means x
must be negative.
If x is negative, then
x is definitely less than 3.
Since we can answer the
REPHRASED target question with certainty, statement 2 is SUFFICIENT
Answer =
B
Cheers,
Brent
