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m09q14

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m09q14 [#permalink] New post 12 Jul 2011, 09:45
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Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
what is 987*987?

974,169
974,219
974,549
975,019
975,369

I solved this using 987*(1000-(10+3)) = 987000 - 12831 = 974169.

OA is calculated using (a+b) * (a-b) + b^2 = a^2. this equals (987+13)*(987-13)+ 13^2

Can someone explain how the quadratic eqn was applied in this question?
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Re: m09q14 [#permalink] New post 12 Jul 2011, 11:46
Simplest way:

987*987
= (1000 - 13)^2
= 1000^2 - 2*1000*13 + 169
= 1000000 - 26000 + 169
= 974000 + 169

Answer is A
Re: m09q14   [#permalink] 12 Jul 2011, 11:46
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