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If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ?

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Senior Manager
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If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  25 Dec 2012, 23:36
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62% (02:38) correct 37% (01:42) wrong based on 54 sessions
If u(u+v)\neq{0} and u >0, is \frac{1}{(u+v)} < \frac{1}{u} + v?

(1) u+v >0
(2) v>0
[Reveal] Spoiler: OA

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Math Expert
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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  26 Dec 2012, 01:42
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If u(u+v)\neq{0} and u >0, is \frac{1}{(u+v)} < \frac{1}{u} + v?

Is \frac{1}{(u+v)} < \frac{1}{u} + v? --> is \frac{-v}{u(u+v)} <v?

(1) u+v >0. Since u+v >0 and u >0, then u(u+v)>0. Now, if v>0, then \frac{-v}{u(u+v)}<0 <v but if v\leq{0}, then \frac{-v}{u(u+v)}\geq{0}\geq{v}. Not sufficient.

(2) v>0. Since u >0 and v>0, then \frac{-v}{u(u+v)}<0<v. Sufficient.

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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  26 Dec 2012, 05:40
PraPon wrote:
If u(u+v)\neq{0} and u >0, is \frac{1}{(u+v)} < \frac{1}{u} + v?

(1) u+v >0
(2) v>0

is \frac{1}{(u+v)} < \frac{1}{u} +vor

is (u+v) > \frac{u}{(1+uv)}

From (1), (u+v) is positive. So assume v is positive . If v is positive LHS is greater. Assume v=0. If v=0, then both are equal.So not sufficient.

We simply see that since v>0 andu>0, \frac{1}{u} > \frac{1}{(u+v)}. So obviously the RHS is greater from (2) alone.

Note: with u positive, saying v is positive is more restrictive than saying (u+v) is positive because in the latter V can be positive or negative. So it is an indication that the answer is likely B.
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Manager
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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  01 Jan 2013, 06:41
Hi Bunuel, if I further simplify your simplified question, i.e. -v/u(u+v) = v to.. by cancelling v on both sides... -1/u(u+v) < 1, then cross multiplying u, given that it is positive the sign does not change...and the question becomes is -1/u+v < u.. ? then I get answer from the condition A as well. given that LHS is negative.. it will be always that u, given that u>0.. what am I doing here wrong? can't I cancel v?
Math Expert
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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  02 Jan 2013, 03:20
Expert's post
pavanpuneet wrote:
Hi Bunuel, if I further simplify your simplified question, i.e. -v/u(u+v) = v to.. by cancelling v on both sides... -1/u(u+v) < 1, then cross multiplying u, given that it is positive the sign does not change...and the question becomes is -1/u+v < u.. ? then I get answer from the condition A as well. given that LHS is negative.. it will be always that u, given that u>0.. what am I doing here wrong? can't I cancel v?

Never multiply (or reduce) an inequality by variable (or by an expression with variable) if you don't know its sign.

So you cannot divide both parts of inequality -v/u(u+v)<v by v as you don't know the sign of this unknown: if v>0 you should write -1/u(u+v)>1 BUT if v<0 you should write -1/u(u+v) >1.

Hope it helps.
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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  03 Jan 2013, 21:21
The ques gets reduced to -> uv(u+v) + v > 0?
if u and v both are + then the above is true , hence B
Manager
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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  25 Mar 2013, 15:43
Bunuel wrote:
If u(u+v)\neq{0} and u >0, is \frac{1}{(u+v)} < \frac{1}{u} + v?

Is \frac{1}{(u+v)} < \frac{1}{u} + v? --> is \frac{-v}{u(u+v)} <v?

(1) u+v >0. Since u+v >0 and u >0, then u(u+v)>0. Now, if v>0, then \frac{-v}{u(u+v)}<0 <v but if v\leq{0}, then \frac{-v}{u(u+v)}\geq{0}\geq{v}. Not sufficient.

(2) v>0. Since u >0 and v>0, then \frac{-v}{u(u+v)}<0<v. Sufficient.

Bunuel, how did you know to subtract 1/u first? For example, why didnt you add 1/u +v on the right hand side first? When I did this it became extremely messy and over two minutes so I had to guess, but I was hoping to be able to avoid this next time and easily see which form it needs to be in. Thanks for your help!
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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink]  25 Mar 2013, 20:24
5
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Expert's post
PraPon wrote:
If u(u+v)\neq{0} and u >0, is \frac{1}{(u+v)} < \frac{1}{u} + v?

(1) u+v >0
(2) v>0

Good Question. This is why GMAT is hard. The concepts are very simple but people get lost while trying to apply them. Let's try to think logically here.

u(u+v)\neq{0} implies that neither u nor (u + v) is 0.
u >0

Take statement 2 first since it is simpler:
(2) v>0

Consider \frac{1}{(u+v)} < \frac{1}{u} + v
If v is positive, \frac{1}{(u+v)} is less than \frac{1}{u} whereas \frac{1}{u} + v is greater than \frac{1}{u}.
Hence right hand side is always greater. Sufficient.

(1) u+v >0

Since u is positive, v can be either 'positive' or 'negative but smaller absolute value than u'.
We have already seen the 'v is positive' case. In that case, right hand side is greater than left hand side.
Let's focus on 'v is negative but smaller absolute value than u'. Say u is 4 and v is -3. Right hand side becomes negative while left hand side is positive. Hence right hand side is smaller.
Not sufficient.

(Edited)
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Save $100 on Veritas Prep GMAT Courses And Admissions Consulting Enroll now. Pay later. Take advantage of Veritas Prep's flexible payment plan options. Veritas Prep Reviews Manager Joined: 21 Jul 2012 Posts: 61 Followers: 0 Kudos [?]: 6 [0], given: 32 Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink] 26 Mar 2013, 04:55 VeritasPrepKarishma wrote: PraPon wrote: If u(u+v)\neq{0} and u >0, is \frac{1}{(u+v)} < \frac{1}{u} + v? (1) u+v >0 (2) v>0 Good Question. This is why GMAT is hard. The concepts are very simple but people get lost while trying to apply them. Let's try to think logically here. u(u+v)\neq{0} implies that neither u nor (u + v) is 0 hence v is also not 0. u >0 Take statement 2 first since it is simpler: (2) v>0 Consider \frac{1}{(u+v)} < \frac{1}{u} + v If v is positive, \frac{1}{(u+v)} is less than \frac{1}{u} whereas \frac{1}{u} + v is greater than \frac{1}{u}. Hence right hand side is always greater. Sufficient. (1) u+v >0 Since u is positive, v can be either 'positive' or 'negative but smaller absolute value than u'. We have already seen the 'v is positive' case. In that case, right hand side is greater than left hand side. Let's focus on 'v is negative but smaller absolute value than u'. Say u is 4 and v is -3. Right hand side becomes negative while left hand side is positive. Hence right hand side is smaller. Not sufficient. Answer (B) u(u+v)\neq{0} implies that neither u nor (u + v) is 0 hence v is also not 0. I am confused why v cannot be 0? if u=5 and v=0, then 5(5+0) does not equal 0 and v=0 and it seems to meet the conditions? Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 4028 Location: Pune, India Followers: 857 Kudos [?]: 3612 [0], given: 144 Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ? [#permalink] 27 Mar 2013, 19:56 Expert's post jmuduke08 wrote: VeritasPrepKarishma wrote: PraPon wrote: If u(u+v)\neq{0} and u >0, is \frac{1}{(u+v)} < \frac{1}{u} + v? (1) u+v >0 (2) v>0 Good Question. This is why GMAT is hard. The concepts are very simple but people get lost while trying to apply them. Let's try to think logically here. u(u+v)\neq{0} implies that neither u nor (u + v) is 0 hence v is also not 0. u >0 Take statement 2 first since it is simpler: (2) v>0 Consider \frac{1}{(u+v)} < \frac{1}{u} + v If v is positive, \frac{1}{(u+v)} is less than \frac{1}{u} whereas \frac{1}{u} + v is greater than \frac{1}{u}. Hence right hand side is always greater. Sufficient. (1) u+v >0 Since u is positive, v can be either 'positive' or 'negative but smaller absolute value than u'. We have already seen the 'v is positive' case. In that case, right hand side is greater than left hand side. Let's focus on 'v is negative but smaller absolute value than u'. Say u is 4 and v is -3. Right hand side becomes negative while left hand side is positive. Hence right hand side is smaller. Not sufficient. Answer (B) u(u+v)\neq{0} implies that neither u nor (u + v) is 0 hence v is also not 0. I am confused why v cannot be 0? if u=5 and v=0, then 5(5+0) does not equal 0 and v=0 and it seems to meet the conditions? Actually, there is no reason why v shouldn't be 0. The only thing is that (u + v) should not be 0. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Save$100 on Veritas Prep GMAT Courses And Admissions Consulting
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Re: If u(u+v) <> 0 and u > 0 is 1/(u + v) < 1/u + v ?   [#permalink] 27 Mar 2013, 19:56
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