BrentGMATPrepNow wrote:
Is x < -1?
(1) (x - 5)² > (1 - x)²
(2) (2 - x)² < (4 + x)²
No takers?
Here's my solution...
Target question: Is x < -1? Statement 1: (x - 5)² > (1 - x)² Note: I created this question to see if anyone falls for the trap of taking the square root of both sides to get: x - 5 > 1 - x
If you fall for this trap, you will incorrectly conclude that x > 3, when, in actuality, we should get x < 3
Key concept: If k is a negative value, and we square it, and then take the square root, we get a positive value.
For example, if x = -5, then x² = (-5)² = 25, which means √x = √25 = 5, which is not the same value we started with. Here's how we should tackle the question....
Given: (x - 5)² > (1 - x)²
Expand and simplify both sides to get: x² - 10x + 25 > 1 - 2x + x²
Subtract x² from both sides: -10x + 25 > 1 - 2x
Add 10x to both sides: 25 > 1 + 8x
Subtract 1 from both sides: 24 > 8x
Divide both sides by 8 to get: 3 > x
So, x could equal 2 in which case the answer to the target question is
NO, x is not less than -1Alternatively, x could equal -3 in which case the answer to the target question is
YES, x is less than -1Since we can’t answer the
target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: (2 - x)² < (4 + x)²Expand and simplify both sides to get: 4 - 4x + x² < 16 + 8x + x²
Subtract x² from both sides to get: 4 - 4x < 16 + 8x
Add 4x to both sides: 4 < 16 + 12x
Subtract 16 from both sides: -12 < 12x
Divide both sides by 12 to get: -1 < x
So, the answer to the target question is
NO, x is not less than -1Since we can answer the
target question with certainty, statement 2 is SUFFICIENT
Answer: B _________________
Brent Hanneson – Creator of gmatprepnow.com
I’ve spent the last 20 years helping students overcome their difficulties with GMAT math, and the biggest thing I’ve learned is…
Many students fail to maximize their quant score NOT because they lack the skills to solve certain questions but because they don’t understand what the GMAT is truly testing -
Learn more