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by vineetbatra » Mon Nov 09, 2009 6:49 pm
(root(9 + root(80)) + root(9 - root(80)))^2.

OA is 20

My question is when I see a question like this I am not sure if I should try to FOIL it or try to Add or Subtract it. Can some share what is the fastest way to attack a question like this.

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by sanjana » Mon Nov 09, 2009 7:02 pm
Whenever you see anything of the form (a+b)^2 or a^2-b^2 just try and factorise using the formula. This will almost always eliminate the root in the question.

For this question you just need to apply (a+b)^2

you get,

9+√80 + 9-√80 + 2√(9+√80)(9-√80)

=18+2√(81-80) --> applying a^2-b^2 = (a+b)(a-b)
=18+2 = 20
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