What is the range of all the roots of |X²-2| = X
A. 4
B. 3
C. 2
D. 1
E. 0
GCT Absolute Value 2
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- richachampion
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OA: D
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Since the absolute value on the left side cannot be equal to a negative value, the right side of the equation must be NONNEGATIVE.richachampion wrote:What is the range of all the roots of |X²-2| = X
A. 4
B. 3
C. 2
D. 1
E. 0
Thus, only nonnegative values for x are viable here.
Case 1: x²-2 = x
x² - x - 2 = 0
(x-2)(x+1) = 0.
x=2 or x=-1.
Since x must be nonnegative, only x=2 is viable.
Case 2: x²-2 = -x
x² + x - 2 = 0
(x+2)(x-1) = 0.
x=-2 or x=1.
Since x must be nonnegative, only x=1 is viable.
The range of the two roots = greater root - smaller root = 2-1 = 1.
The correct answer is D.
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When solving equations involving ABSOLUTE VALUE, there are 3 steps:richachampion wrote:What is the range of all the roots of |x² - 2| = x
A. 4
B. 3
C. 2
D. 1
E. 0
1. Apply the rule that says: If |x| = k, then x = k and/or x = -k
2. Solve the resulting equations
3. Plug solutions into original equation to check for extraneous roots
EITHER x² - 2 = x OR x² - 2 = -x
x² - 2 = x
Rearrange: x² - x - 2 = 0
Factor: (x - 2)(x + 1) = 0
Solve: x = 2 or x = -1
x² - 2 = -x
Rearrange: x² + x - 2 = 0
Factor: (x + 2)(x - 1) = 0
Solve: x = -2 or x = 1
Plug solutions into original equation to check for extraneous roots....
Plug in x = 2 to get |2² - 2| = 2
Evaluate: |2| = 2 WORKS!
So, x = 2 is a solution
Plug in x = -1 to get |(-1)² - 2| = -1
Evaluate: |-1| = -1 DOESN'T WORK
So, x = -1 is a NOT solution
Plug in x = -2 to get |(-2)² - 2| = -2
Evaluate: |2| = -2 DOESN'T WORK
So, x = -2 is a NOT solution
Plug solutions into original equation to check for extraneous roots....
Plug in x = 1 to get |1² - 2| = 1
Evaluate: |-1| = 1 WORKS!
So, x = 1 is a solution
So, the only two valid solutions are x = 2 and x = 1
So, the range is 1
Answer: D
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Hi richachampion,
This question can be solved rather easily with some 'brute force' math. The answer choices are all small integers, so we know that the range can't be very big. Chances are good that we can just TEST VALUES until we find the range.
With |X^2 - 2| = X we know that X cannot be negative (since the result of an absolute value calculation can't be negative). So let's get to work TESTing small integers - we really just have to take notes and look for a pattern.
IF...
X = 0, we get |-2| = 0, which is NOT correct.
X = 1, we get |-1| = 1, so X COULD be 1
X = 2, we get |-2| = 2, so X COULD be 2
X = 3, we get |7| = 3, which is NOT correct.
If we increase the value of X, then we'll just end up with an absolute value that is farther away from the value of X, so we can stop working here. For the sake of thoroughness, you might consider TESTing non-integers (for example, X = 1/2), but you'll rather quickly see that the equation won't balance out (the denominators will be different)...
IF...
X = 1/2, we get |-7/4| = 1/2, which is NOT correct.
Thus, the only solutions are 1 and 2, so the range is 1.
Final Answer: D
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This question can be solved rather easily with some 'brute force' math. The answer choices are all small integers, so we know that the range can't be very big. Chances are good that we can just TEST VALUES until we find the range.
With |X^2 - 2| = X we know that X cannot be negative (since the result of an absolute value calculation can't be negative). So let's get to work TESTing small integers - we really just have to take notes and look for a pattern.
IF...
X = 0, we get |-2| = 0, which is NOT correct.
X = 1, we get |-1| = 1, so X COULD be 1
X = 2, we get |-2| = 2, so X COULD be 2
X = 3, we get |7| = 3, which is NOT correct.
If we increase the value of X, then we'll just end up with an absolute value that is farther away from the value of X, so we can stop working here. For the sake of thoroughness, you might consider TESTing non-integers (for example, X = 1/2), but you'll rather quickly see that the equation won't balance out (the denominators will be different)...
IF...
X = 1/2, we get |-7/4| = 1/2, which is NOT correct.
Thus, the only solutions are 1 and 2, so the range is 1.
Final Answer: D
GMAT assassins aren't born, they're made,
Rich
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|x² - 2| = x
We know that |z| is always = √z², so let's write it that way:
√(x² - 2)² = x
then square both sides:
(x² - 2)² = x²
then expand:
x� - 4x² + 4 = x²
then solve:
x� - 5x² + 4 = 0
(x² - 4) * (x² - 1) = 0
Which gives us
(x² - 4) = 0 or (x² - 1) = 0
and we know that |x² - 2| gives a positive result, so x must be positive.
That means (x² - 4) only has the solution x = 2, and (x² - 1) = 0 only has the solution x = 1.
That gives us our two solutions, and we're done!
We know that |z| is always = √z², so let's write it that way:
√(x² - 2)² = x
then square both sides:
(x² - 2)² = x²
then expand:
x� - 4x² + 4 = x²
then solve:
x� - 5x² + 4 = 0
(x² - 4) * (x² - 1) = 0
Which gives us
(x² - 4) = 0 or (x² - 1) = 0
and we know that |x² - 2| gives a positive result, so x must be positive.
That means (x² - 4) only has the solution x = 2, and (x² - 1) = 0 only has the solution x = 1.
That gives us our two solutions, and we're done!
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Sir, can you please explain above with example?Since the absolute value on the left side cannot be equal to a negative value, the right side of the equation must be NONNEGATIVE.
Thus, only nonnegative values for x are viable here.
Thanks,
Sid
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Hi Sid,
Absolute value calculations turn negative results into positive ones.
For example, if we're dealing with |X - 3|...
IF....
X = -1, then | -1 - 3| = |-4| = 4
X = 2, then |2 - 3| = |-1| = 1
X = 3, then |3 - 3| = |0| = 0
X = 4, then |4 - 3| = |1| = 1
Using that knowledge, we know that |X^2 - 2| can NEVER end in a negative result (it could end in a 0 result, but that would be the minimum possible result).
GMAT assassins aren't born, they're made,
Rich
Absolute value calculations turn negative results into positive ones.
For example, if we're dealing with |X - 3|...
IF....
X = -1, then | -1 - 3| = |-4| = 4
X = 2, then |2 - 3| = |-1| = 1
X = 3, then |3 - 3| = |0| = 0
X = 4, then |4 - 3| = |1| = 1
Using that knowledge, we know that |X^2 - 2| can NEVER end in a negative result (it could end in a 0 result, but that would be the minimum possible result).
GMAT assassins aren't born, they're made,
Rich