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Squaring

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Source: — Problem Solving |
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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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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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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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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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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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