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by kwah » Sun Apr 01, 2012 6:47 pm
Attached is a question from GMAT Prep Test #2.

What is the most efficient way of achieving the result?

Answer: E

Thanks,
K
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by Anurag@Gurome » Sun Apr 01, 2012 7:19 pm
kwah wrote:Attached is a question from GMAT Prep Test #2.

What is the most efficient way of achieving the result?

Answer: E

Thanks,
K
Apply the formula, (a + b)² = a² + 2ab + b²
a = √(9 + √80) and b = √(9 - √80)
a² = 9 + √80, b² = 9 - √80, and
2ab = 2 * √(9 + √80) * √(9 - √80) = 2√[(9 + √80)(9 - √80)] = 2√81 - 80) = 2√1 = 2

So, [√(9 + √80) + √(9 - √80)]² = 9 + √80 + 2 + 9 - √80 = 20

The correct answer is E.
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