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| OG - DS - #139 |
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Dan Bernstein
MGMAT STAFF
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dk,
Fractions cannot be reduced in the manner you describe when dealing with the operations of addition and sutraction. Try substituting numbers for the x and y and you will quickly realize this. For example, 2/(2+3) equals 2/5 rather than 1/3, the answer that would result if the 2's were simply eliminated. My approach to this particular problem was one of plugging in numbers. Beginning with statement (1) and our AD/BCE grid, x>0 but we have no idea about the value of y. Eliminate AD from the AD/BCE grid. Statement (2) tells us that y<0, but we have no idea about the value of x. Eliminate B from the AD/BCE grid. Together we know that x>0 and y<0, but we don't know about the absolute value of either number. For example, if x=2 and y=-1, the value of (x-y)/(x+y) = 3. However, if x=2 and y=-3, the value of (x-y)(x+y) = 5/-1, or -5. Since 3>1 but -5<1, the correct answer is E.
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| Question re: Rephrasal |
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Guest
Guest
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so is it incorrect to rephrase this statement to:
Is (x-y) > (x+y) ? If this rephrasal is appropriate....is there a value for x that one could find that would make (x-y) less than (x+y) when y is negative? (I) If y is negative, and x positive then x-y will be greater than x+y, since you're subtracting from a negative number - and by definition adding (II) If y is negative, and x negative, then x-y will be greater than x+y, because of the same principle as above... I.e. I used this rephrasal and got B. I am clearly doing something incorrectly here - is my rephrasal off-base? Or are my two postulations incorrect - If so, can you provide counter-examples? ' Thanks. |
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Stacey Koprince
MGMAT STAFF
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It is incomplete (not incorrect exactly) to rephrase the question as (x-y) > (x+y) because we don't know whether (x+y) is positive or negative.
Starting with: x-y/x+y > 1 In order to move (x+y) to the right hand side of the equation, you have to multiply. When you multiply or divide an inequality by a negative, you have to switch the sign. Because we don't know whether (x+y) is positive or negative, we have to test BOTH possibilities: If x+y is positive, then x-y > x+y If x+y is negative, then x-y < x+y You can keep going this way, but this suddenly makes the problem a lot more complicated, which is why Dan tried real numbers with this one. |
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| OG - DS - #139 |
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