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sarahw_gmat: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Fri Mar 12, 2010 1:39 pm; Thanked: 1 times

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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Source: Beat The GMAT — Data Sufficiency | Tue Jul 19, 2011 11:29 am

clock60: Legendary Member; Posts: 759; Joined: Mon Apr 26, 2010 10:15 amFollowed by: 3 members; Thanked: 85 times

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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sarahw_gmat: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Fri Mar 12, 2010 1:39 pm; Thanked: 1 times

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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clock60: Legendary Member; Posts: 759; Joined: Mon Apr 26, 2010 10:15 amFollowed by: 3 members; Thanked: 85 times

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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