Squaring

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medea66: Master | Next Rank: 500 Posts; Posts: 186; Joined: Sat Jan 26, 2008 10:05 pm; Thanked: 2 times

by medea66 » Mon Aug 11, 2008 8:00 pm

I picked numbers for this one(perfect squares), but I guess not the right numbers since they don't work out with the given solution.......

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Source: Beat The GMAT — Problem Solving | Mon Aug 11, 2008 8:00 pm

sudhir3127: Legendary Member; Posts: 829; Joined: Mon Jul 07, 2008 10:09 pmFollowed by: 3 members; Location: INDIA; Thanked: 84 times

by sudhir3127 » Mon Aug 11, 2008 8:06 pm

Plug in the values...
i took a=2 and b =1

hence sqroot(a^2-b^2) = sqrt(2^2-1^2)=sqrt(3)

only option D gives me sqrt(3).

hope it helps..

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bluementor: Master | Next Rank: 500 Posts; Posts: 418; Joined: Wed Jun 11, 2008 5:29 am; Thanked: 65 times

by bluementor » Tue Aug 12, 2008 5:45 am

I'm not sure if my approach would be ideal for other similar questions, but here it goes:

sqrt(a^2 - b^2)
= sqrt((a-b)(a+b))
= sqrt(a-b)*sqrt(a+b)

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santa_dem: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Fri Aug 08, 2008 4:44 am; Thanked: 4 times; GMAT Score:750

by santa_dem » Tue Aug 12, 2008 5:52 am

Just remember that x^2-y^2=(x+y)*(x-y).

Simple as that.

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medea66: Master | Next Rank: 500 Posts; Posts: 186; Joined: Sat Jan 26, 2008 10:05 pm; Thanked: 2 times

by medea66 » Tue Aug 12, 2008 5:56 pm

I think the mistake I made is that I felt compelled to pick perfect squares for A & B, and in the process I complicated the calculation.

The calculation is alot simpler if I pick just 1 & 2 for A & B, respectively.

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Tue Aug 12, 2008 7:57 pm

santa_dem wrote:Just remember that x^2-y^2=(x+y)*(x-y).

Simple as that.

Exactly. You will see a difference of squares on your GMAT, guaranteed. Be sure you can recognize it, and factor it when it shows up. If you notice, in this question, that the expression under the root is a difference of squares, the problem is straightforward- much faster than choosing numbers.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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