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Bunuel
What is the sum of 9,999^2 and 10,001^2?

(A) 99,980,001
(B) 199,999,998
(C) 200,000,002
(D) 399,999,996
(E) 400,000,004

Two ways..

LOGICAL using number properties and choices..
\(9,999^2+ 10,001^2\)
UNITS digit \(= 9^2+1^2=81+01=2\)
ONLY C has a units digit of 2

Algebraic..
\(9,999^2=(10000-1)^2\) and \(10001^2=(10,000+1)^2\)
Now two formulas are
1) \((a-1)^2=a^2-2a+1\)
2) \((a+1)^2=a^2+2a+1\)
Add both the above cases
\((a-1)^2+(a+1)^2=a^2+2a+1+a^2-2a+1=2(a^2+1)\)..Here a=10000
so \(2(a^2+1)=2(10000^2+1)=2*100000001=200000002\)

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