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[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/session.php on line 580: sizeof(): Parameter must be an array or an object that implements Countable
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/session.php on line 636: sizeof(): Parameter must be an array or an object that implements Countable
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10050010040

Posted: Mon Oct 16, 2017 6:59 pm

by ada

$1^2+2^2+3^2+4^2+\dotsb+n^2=\frac{n(n+1)(2n+1)}{6}$