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(112 + 122 + 132 + ... + 202) =?

Correct Answer: 2485

Explanation:

(11 2 + 12 2 + 13 2 + ... + 20 2 ) = (1 2 + 2 2 + 3 2 + ... + 20 2 ) - (1 2 + 2 2 + 3 2 + ... + 10 2 )


[ Ref: (12 + 22 + 32 + ... + n2) = 1 n(n + 1)(2n + 1) ]    
6


20 x 21 x 41 - 10 x 11 x 21
6 6

= (2870 - 385)


= 2485.


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