9870

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Number

$9870$ (nine thousand, eight hundred and seventy) is:

$2 \times 3 \times 5 \times 7 \times 47$

The $140$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $8911$, $9045$, $9180$, $9316$, $9453$, $9591$, $9730$:

$9870 = \ds \sum_{k \mathop = 1}^{140} k = \dfrac {140 \times \paren {140 + 1} } 2$

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