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What is the value of p^3 + 2p ? (1) p^2 5p + 4 = 0. (2) The

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What is the value of p^3 + 2p ? (1) p^2 5p + 4 = 0. (2) The [#permalink]

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11 Jun 2010, 09:35
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What is the value of p^3 + 2p ?
(1) p^2 − 5p + 4 = 0.
(2) The value of p2 + 1/p is –2

OA will be posted later
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11 Jun 2010, 10:01
IMO B

A is INSUFFICIENT. P is 1 or 4.
B is SUFFICIENT. Value is -1.
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11 Jun 2010, 11:01
hardnstrong wrote:
What is the value of p^3 + 2p ?
(1) p^2 − 5p + 4 = 0.
(2) The value of p2 + 1/p is –2

OA will be posted later

Not a good question.

Question: $$p^3 + 2p=?$$

(1) $$p^2-5p+4=0$$ --> $$p=1$$ or $$p=4$$. Two values for $$p^3 + 2p$$. Not sufficient.

(2) If statement 2 is $$\frac{p^2+1}{p}=-2$$, then $$p^2+2p+1=0$$ --> $$p=-1$$ --> one value for $$p$$ --> $$p^3 + 2p=-3$$. Sufficient.
BUT if this is true, than two statements contradict (from 1: $$p=1$$ or $$p=4$$ and from 2: $$p=-1$$) and in DS statements never contradict.

If statement 2 is $$p^2+\frac{1}{p}=-2$$, then $$p^3+1=-2p$$ --> $$p^3 + 2p=-1$$, directly gives the value of $$p^3 + 2p$$. Sufficient.
BUT again if we take the values from 1 ($$p=1$$ or $$p=4$$) they won't satisfy the equation $$p^2+\frac{1}{p}=-2$$ and in DS both statements must be true.

So I guess the answer for this question is B, but you'll never see such question on real GMAT.

Hope it's clear.
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11 Jun 2010, 21:47
stmt 2 is p^2 + (1/p) = -2

i missed that it directly gives the value of p^3 +2p and was trying to solve it for p

Thanks for the explanation
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Re: equation problem   [#permalink] 11 Jun 2010, 21:47
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