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I got this correct through backsolving....but didn't get it

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Senior Manager
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I got this correct through backsolving....but didn't get it [#permalink] New post 09 Feb 2008, 13:47
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I got this correct through backsolving....but didn't get it through direct method...
Here is what i tried:

(90/v-3) - (90/v+3) = 0.5
= > {90*(v+3)} - {90*(v-3)}/ v^2 - 3^2 = 0.5

and it is not working :roll:
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Re: PS: Prep [#permalink] New post 09 Feb 2008, 13:56
are you solving for v?

btw, you will get v=33 (v^2 = 1089) and answer should be 2.5
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Re: PS: Prep [#permalink] New post 10 Feb 2008, 09:54
yup, I was trying to get the answer through v..I was pretty late last night..got it right in the morning. thx
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Re: PS: Prep [#permalink] New post 10 Feb 2008, 20:52
rishi2377 wrote:
I got this correct through backsolving....but didn't get it through direct method...
Here is what i tried:

(90/v-3) - (90/v+3) = 0.5
= > {90*(v+3)} - {90*(v-3)}/ v^2 - 3^2 = 0.5

and it is not working :roll:
Attachment:
New Microsoft Word Document.doc


Man this was kindof a nasty lil problem. Easy conceptually, but lots of busy work.

(v-3)(t+1/2)=90
(v+3)(t)=90

vt+v/2-3t-3/2=90
vt+3t

2vt+v-6t=183
-2(vt+3)=90(-2) ---> v=12t+3

(12t+6)(t)=90 ----> t^2+(t/2)^2-15/2=0 ---> (t+3)(t-5/2)=0 so t=2.5

i think a better approach would be to just plug in the answer choices.
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Re: PS: Prep [#permalink] New post 12 Feb 2008, 06:06
GMATBLACKBELT wrote:
rishi2377 wrote:
I got this correct through backsolving....but didn't get it through direct method...
Here is what i tried:

(90/v-3) - (90/v+3) = 0.5
= > {90*(v+3)} - {90*(v-3)}/ v^2 - 3^2 = 0.5

and it is not working :roll:
Attachment:
New Microsoft Word Document.doc


Man this was kindof a nasty lil problem. Easy conceptually, but lots of busy work.

(v-3)(t+1/2)=90
(v+3)(t)=90

vt+v/2-3t-3/2=90
vt+3t

2vt+v-6t=183
-2(vt+3)=90(-2) ---> v=12t+3

(12t+6)(t)=90 ----> t^2+(t/2)^2-15/2=0 ---> (t+3)(t-5/2)=0 so t=2.5

i think a better approach would be to just plug in the answer choices.



lol yes black belt...I got it correct through backsolving but tried to get it through direct method and gu6ess what, it was 0315 in the morning. lol thx anyway.
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Re: PS: Prep [#permalink] New post 12 Feb 2008, 06:13
rishi2377 wrote:
I got this correct through backsolving....but didn't get it through direct method...
Here is what i tried:

(90/v-3) - (90/v+3) = 0.5
= > {90*(v+3)} - {90*(v-3)}/ v^2 - 3^2 = 0.5

and it is not working :roll:
Attachment:
New Microsoft Word Document.doc



Looks good so far! Rather that factoring to get v^2=1089, we can do the following:
Multiply be 2 to get 180/(v-3) - 180/(v+3) = 1

Look for two factors of 180 = 2^2*3^2*5 that differ by 6: 30 and 36. Thus v=33
Re: PS: Prep   [#permalink] 12 Feb 2008, 06:13
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I got this correct through backsolving....but didn't get it

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