og ps 111

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resilient: Legendary Member; Posts: 789; Joined: Sun May 06, 2007 1:25 amFollowed by: 6 members; Location: Southern California, USA; Thanked: 15 times

by resilient » Thu Feb 07, 2008 9:37 pm

if s>0 and root r/s = s, what is r in terms of s?

a. 1/s
b. root s
c. s root s
d. s^3
e. s^2-s

I tried picking numbers but I could figure out what to put in for the values!

qa is d

Appetite for 700 and I scraped my plate!

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Source: Beat The GMAT — Problem Solving | Thu Feb 07, 2008 9:37 pm

Saffa: Senior | Next Rank: 100 Posts; Posts: 41; Joined: Thu Jul 12, 2007 1:16 am; Location: Cape Town South Africa; Thanked: 5 times

by Saffa » Fri Feb 08, 2008 3:03 am

root r/s = s

square botn sides

r/s = s^2

multiply both sides by s

r = s^3

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sibbineni: Master | Next Rank: 500 Posts; Posts: 222; Joined: Fri Jan 04, 2008 8:10 pm; Thanked: 15 times

by sibbineni » Fri Feb 08, 2008 4:37 pm

sqrt(r/s)=s

Squaring on both sides

r/s=s^2

cross multiplying we have

r=s^3

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gmatguy16: Master | Next Rank: 500 Posts; Posts: 124; Joined: Thu Aug 23, 2007 5:11 am; Thanked: 2 times

by gmatguy16 » Sat Feb 09, 2008 10:34 am

please include brackets in questions as much as possible root r/s could be interpreted as root(r)/s or root(r/s).of course in this case it was simple to figure out but other complex questions it does help.

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sfiqal: Newbie | Next Rank: 10 Posts; Posts: 4; Joined: Wed May 20, 2015 12:24 pm; Location: United Kingdom

by sfiqal » Thu May 19, 2016 12:21 pm

For this question, initially I got a C. I did it this way:

squareroot (r/s) = s

squareroot both side and get

r/s = squareroot s

and finally

r= s squareroot (s)

Okay, I now know that this is wrong but why is this way wrong? Why can't we squareroot both sides (instead of squaring both sides which is the the right way)?

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GMAT/MBA Expert

[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Thu May 19, 2016 3:34 pm

Hi sfiqal,

From a 'math' standpoint, you can do whatever you like to both sides of an equation. However, when you square-root a calculation that has already been square-rooted, that 'act' does NOT cancel out the square-root... it turns the result into a quad-root.

For example, if you have the square-root of 16, then take the square-root of THAT number, you have...

Square-root of 16 = 4.... square-root of 4 = 2.....

Thus, the quad-root of 16 = 2

To "undo" a square-root, you have to SQUARE both sides of the equation, just as the correct answer does in this question.

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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