7875

Number
$7875$ (seven thousand, eight hundred and seventy-five) is:


 * $3^2 \times 5^3 \times 7$


 * The $125$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $6786$, $6903$, $7021$, $7140$, $7260$, $7381$, $7503$, $7626$, $7750$:
 * $7750 = \displaystyle \sum_{k \mathop = 1}^{125} k = \dfrac {125 \times \left({125 + 1}\right)} 2$


 * The $63$rd hexagonal number after $1$, $6$, $15$, $28$, $45$, $66$, $91$, $\ldots$, $5565$, $5778$, $5995$, $6216$, $6441$, $6670$, $6903$, $7140$, $7381$, $7626$:
 * $7875 = \displaystyle \sum_{k \mathop = 1}^{63} \left({4 k - 3}\right) = 63 \left({2 \times 63 - 1}\right)$