9730

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Number

$9730$ (nine thousand, seven hundred and thirty) is:

$2 \times 5 \times 7 \times 139$

The $70$th hexagonal number after $1$, $6$, $15$, $28$, $45$, $66$, $91$, $\ldots$, $8128$, $8385$, $8646$, $8911$, $9180$, $9453$:

$9730 = \ds \sum_{k \mathop = 1}^{70} \paren {4 k - 3} = 70 \paren {2 \times 70 - 1}$

The $139$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $8911$, $9045$, $9180$, $9316$, $9453$, $9591$:

$9730 = \ds \sum_{k \mathop = 1}^{139} k = \dfrac {139 \times \paren {139 + 1} } 2$

Also see

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