820

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Number

$820$ (eight hundred and twenty) is:

$2^2 \times 5 \times 41$

The $40$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $325$, $351$, $378$, $406$, $435$, $465$, $496$, $528$, $561$, $595$, $630$, $666$, $703$, $741$, $780$:

$820 = \ds \sum_{k \mathop = 1}^{40} k = \dfrac {40 \times \paren {40 + 1} } 2$

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