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Re: If pq 0, is p + q > 1/p + 1/q? (1) p < q < 1 (2) pq < 1 [#permalink]
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BengalScientist wrote:
BengalScientist wrote:
Is P + Q > 1/P + 1/ Q?

1) P < Q < 1
2) PQ < 1


I simplified the equation as P + Q > (Q + P)/PQ , PQ>1

Statement 2 is sufficient.

What am i missing here? can some one unfreeze my brain please...


Firstly,you cannot cancel out the factor (P+Q) as because it can be zero also. Secondly, even if (P+Q) is not zero,you cannot cross multiply the term PQ as because you don't know about its sign. When PQ<1, PQ can be positive or negative. Had the 2nd fact statement read as PQ>1, then we could have easily cross multiplied. Thus, the correct inequality to solve would be -->Is (P+Q)(1-1/PQ)>0.

From F.S 1, for P = -1/2 and Q = 1/2, P+Q = 1/P+1/Q = 0. Thus, we get a NO for the question stem. But for P=-1/2 and Q = -1/4, P+Q(-3/4) is greater than 1/P+1/Q(-6). Thus , we get a YES for the question stem. Thus, this fact statement is Insufficient.

From F. S 2, we can take the exact 2 sets of values for P and Q as above and clearly this fact statement is also Insufficient.

Taking both together, we again have no NEW information.Insufficient.

E.
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Re: If pq 0, is p + q > 1/p + 1/q? (1) p < q < 1 (2) pq < 1 [#permalink]
Expert Reply
Hi All,

We're asked if P + Q > (1/P) + (1/Q). This is a YES/NO question. We can answer it by TESTing VALUES, although we'll have to think in terms of some rarer examples (fractions) to get the correct answer.

1) P < Q < 1

IF....
P=1/4, Q=1/2.... then 1/4 + 1/2 is NOT greater than (4) + (2)... and the answer to the question is NO.
P= -1/2, Q= -1/4.... then -1/2 + -1/4 IS greater than (-2) + (-4)... and the answer to the question is YES.
Fact 1 is INSUFFICIENT

2) (P)(Q) < 1
Both of the examples that we used in Fact 1 also 'fit' Fact 2:
IF....
P=1/4, Q=1/2.... then 1/4 + 1/2 is NOT greater than (4) + (2)... and the answer to the question is NO.
P= -1/2, Q= -1/4.... then -1/2 + -1/4 IS greater than (-2) + (-4)... and the answer to the question is YES.
Fact 2 is INSUFFICIENT

Combined, we have already have 2 examples that 'fit' both Facts and produce different answers (one 'NO' and one 'YES'), so no additional work is required.
Combined, INSUFFICIENT

Final Answer:

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Re: If pq 0, is p + q > 1/p + 1/q? (1) p < q < 1 (2) pq < 1 [#permalink]
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instead of going through all these we can always assume p=0 or q=0 or both , then it will not define the cases anyhow..so E
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Re: If pq 0, is p + q > 1/p + 1/q? (1) p < q < 1 (2) pq < 1 [#permalink]
BengalScientist wrote:
Is P + Q > 1/P + 1/ Q?

(1) P < Q < 1
(2) PQ < 1

M21-19

1. I see that two values that can be used prove and disprove both the statements are \(P = \frac{1}{3}\) and \(Q = \frac{1}{2}\) & \(P = \frac{-1}{2}\) and \(Q = \frac{-1}{3}\)
2. Plugging both sets of values into the inequality

3.a \(P + Q = \frac{1}{3} + \frac{1}{2} = \frac{5}{6}\)

3.b \(\frac{1}{P} + \frac{1}{Q} = 3 + 2 = 5\)

3.c In the above case, \(P + Q < \frac{1}{P} + \frac{1}{Q}\)

4.a \(P + Q = \frac{-1}{2} + (\frac{-1}{3}) = \frac{-5}{6}\)

4.b \(\frac{1}{P} + \frac{1}{Q} = -2 + (-3) = -5\)

4.c In the above case, \(P + Q > \frac{1}{P} + \frac{1}{Q}\)

Ans. E
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Re: If pq 0, is p + q > 1/p + 1/q? (1) p < q < 1 (2) pq < 1 [#permalink]
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Is \(p + q > \frac{1}{p} + \frac{1}{q}\) ?

(1) \(p < q < 1\)

If both \(p\) and \(q\) are positive fractions, then the left-hand side is the sum of two numbers less than 1, while the right-hand side is the sum of two numbers greater than 1. Therefore, \(p + q < \frac{1}{p} + \frac{1}{q}\). For instance, consider \(p = \frac{1}{3}\) and \(q = \frac{1}{2}\). However, if both are negative fractions, then the left-hand side will be the sum of two negative numbers greater than -1, whereas the right-hand side will be the sum of two negative numbers less than -1. In this scenario, \(p + q > \frac{1}{p} + \frac{1}{q}\). For instance, consider \(p = -\frac{1}{2}\) and \(q = -\frac{1}{3}\). Not sufficient.

(2) \(pq < 1\)

Using the logic from statement (1), this statement alone also does not determine if \(p + q > \frac{1}{p} + \frac{1}{q}\). Not sufficient.

(1)+(2) We can use the same logic as for (1) and (2). Even when combined, the statements are not sufficient.


Answer: E
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Re: If pq 0, is p + q > 1/p + 1/q? (1) p < q < 1 (2) pq < 1 [#permalink]
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