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Q71 of 74 Page 9

1²+ (1² + 2²) + (1² + 2² + 3²) + ……

a_n = 1² + 2² + … + n² =

=

=

S_n =

=

=

= (n(n+1) + 2n + 1+ 1)

= (n²+n+2n+1+1)

= (n²+3n+2)

= (n+1)(n+2)

=

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