969

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Number

$969$ (nine hundred and sixty-nine) is:

$3 \times 17 \times 19$

The $17$th tetrahedral number, after $1$, $4$, $10$, $20$, $35$, $56$, $84$, $120$, $165$, $220$, $286$, $364$, $455$, $560$, $680$, $816$:

$969 = \ds \sum_{k \mathop = 1}^{17} \frac {k \paren {k + 1} } 2 = \dfrac {17 \paren {17 + 1} \paren {17 + 2} } 6$

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