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Gmat Prep DS

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Junior | Next Rank: 30 Posts
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Gmat Prep DS

by sarahw_gmat » Tue Jul 19, 2011 11:29 am
Is 1/P > r/( r^2 + 2 ) ?

1. P = r
2. r > 0

OA : C

My Ans: A

From the given statement : r^2 + 2 > Pr
1. since P = r
r^2 + 2 > r.r ... which is always true

Please explain what am I doing wrong.

Thanks.
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by clock60 » Tue Jul 19, 2011 11:42 am
for sure you can multiply by (r^2+2) as it is +ve
but you can`t multiply by P is it can be +ve or -ve, and if p is -ve you must reverse the sign of the inequality
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Junior | Next Rank: 30 Posts
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by sarahw_gmat » Tue Jul 19, 2011 11:45 am
clock60 wrote:for sure you can multiply by (r^2+2) as it is +ve
but you can`t multiply by P is it can be +ve or -ve, and if p is -ve you must reverse the sign of the inequality
Thanks a lot. Can you please provide complete solution?
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by clock60 » Tue Jul 19, 2011 12:09 pm
i`ll try
does 1/p>r/(r^2+2). or does
(r^2+2)/p>r
(1)p=r=x
(x^2+2)/x=x+(2/x) vs x, we can cancel x from both parts and arrive does 2/x>0 it is possible if x=p=r>0
but we not given sign
so 1 st insuff
(2) second insuff as it does not mention p
both: p=r=x>0 so 2/x>0 so suff
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