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# m09q14

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Manager
Joined: 23 Jan 2011
Posts: 127
Followers: 1

Kudos [?]: 81 [0], given: 13

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12 Jul 2011, 09:45
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?
Senior Manager
Joined: 28 Jun 2009
Posts: 445
Location: United States (MA)
Followers: 18

Kudos [?]: 176 [0], given: 46

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12 Jul 2011, 11:46
Simplest way:

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

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

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