inequalities
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Source: Beat The GMAT — Data Sufficiency |
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sudharsong
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sudharsong
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raju232007
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statement 1:
x>0
x is positive
no information about y
Insufficient
statement 2:
y<0
y is negative
no information about x
Insufficient
Combining both the statements we get
x is positive and y is negative
Let x=2 & y=-3
(x-y)/(x+y)=-5
Is -5>1 ....Ans is no
Let x=3 & y=-2
(x-y)/(x+y)=5
Is 5>1 ...Ans is yes
So the ans should be E....
x>0
x is positive
no information about y
Insufficient
statement 2:
y<0
y is negative
no information about x
Insufficient
Combining both the statements we get
x is positive and y is negative
Let x=2 & y=-3
(x-y)/(x+y)=-5
Is -5>1 ....Ans is no
Let x=3 & y=-2
(x-y)/(x+y)=5
Is 5>1 ...Ans is yes
So the ans should be E....
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rohangupta83
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msvmuthu
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Why shouldnt we simplify the above given equation?
for example, in the above question I simplified the equation to:
y<0
since the first choice is x>0,
and since we have derived y<0;
(x-y) / (x+y) => will be like (x+y) /(x-y)
there fore surely, this will be greater than 1.
There fore my choice was A.
Please some one throw more light on this!
for example, in the above question I simplified the equation to:
y<0
since the first choice is x>0,
and since we have derived y<0;
(x-y) / (x+y) => will be like (x+y) /(x-y)
there fore surely, this will be greater than 1.
There fore my choice was A.
Please some one throw more light on this!
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Ian Stewart
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There are two issues above- both are common mistakes, so important to understand:msvmuthu wrote:Why shouldnt we simplify the above given equation?
for example, in the above question I simplified the equation to:
y<0
since the first choice is x>0,
and since we have derived y<0;
(x-y) / (x+y) => will be like (x+y) /(x-y)
there fore surely, this will be greater than 1.
There fore my choice was A.
Please some one throw more light on this!
First, you've 'simplified' the inequality to get y < 0, then used this as part of your solution. You can't do this: 'Is (x-y)/(x+y) > 1?' was the question; it's not a fact. If you assume that (x-y)/(x+y) > 1 is true, it shouldn't be surprising that you then discover that (x-y)/(x+y) > 1 is true; this is what's called 'begging the question' in logic. It can be a very good idea to simplify expressions like the one in the question above, but be clear about what you're doing: you are not deriving a fact; instead you are rephrasing the question.
Second, when you have an inequality, if you multiply both sides of the inequality by a negative, you must reverse the inequality. If, say, you know that
a/b > 1
you cannot multiply both sides by b and conclude that a > b, at least not without knowing something about b. It is certainly true that a > b if b is positive, but if b is negative, we would need to reverse the inequality; we would find that a < b. When you rewrote the inequality:
(x-y)/(x+y) > 1
to get y < 0, you were assuming x+y is positive. If x+y is negative, you need to reverse the inequality; you would find that y > 0.
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gkumar
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Ian, can you explain the fallacy in my logic as described here? https://www.beatthegmat.com/x-y-x-y-1-t47013.html
