First & Second-Degree Equations

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BlindVision: Master | Next Rank: 500 Posts; Posts: 253; Joined: Fri Dec 26, 2008 8:39 pmFollowed by: 1 member; Thanked: 8 times

by BlindVision » Sun Mar 08, 2009 4:28 pm

If rs IS NOT equal to 0, is 1/r + 1/s = 4?

1) r + s = 4rs

2) r = s

OA = A

I spent a half-hour trying to figure this one out. Please help

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Source: Beat The GMAT — Data Sufficiency | Sun Mar 08, 2009 4:28 pm

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

Re: First & Second-Degree Equations

by Brent@GMATPrepNow » Sun Mar 08, 2009 6:21 pm

BlindVision wrote:If rs IS NOT equal to 0, is 1/r + 1/s = 4?

1) r + s = 4rs

2) r = s

OA = A
I spent a half-hour trying to figure this one out. Please help

To begin, let's reword the question, by adding 1/r + 1/s to create one rational expression.
Find common denominator: s/rs + r/rs
Add numerators: (s+r)/rs
So, rather than ask "Does 1/r + 1/s = 4?" we can ask "Does (s+r)/rs = 4?"
Even better, we can cross multiply (s+r)/rs = 4/1 to get s+r = 4sr
So, another wording of the question can be "Does s+r = 4sr?"

(1) r + s = 4rs (SUFFICIENT)
(2) Knowing that r=s does not help us determine whether s+r = 4sr (INSUFFICIENT)

Brent Hanneson - Creator of GMATPrepNow.com

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logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

Re: First & Second-Degree Equations

by logitech » Sun Mar 08, 2009 7:07 pm

I always start with rephrasing what is asked and trying to express it as simple as I can:

1/r + 1/s = 4

means: r+s/rs = 4 ; r+s=4rs ?

Statement 1) BINGO
Statement 2) INSUF

Choose (A)

LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"

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BlindVision: Master | Next Rank: 500 Posts; Posts: 253; Joined: Fri Dec 26, 2008 8:39 pmFollowed by: 1 member; Thanked: 8 times

by BlindVision » Sun Mar 08, 2009 8:14 pm

B.H. & Logitech! Thank you... you guys helped me make more sense of it

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