1275

Number
$1275$ (one thousand, two hundred and seventy-five) is:
 * $3 \times 5^2 \times 17$


 * The $50$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $703$, $741$, $780$, $820$, $861$, $903$, $946$, $990$, $1035$, $1081$, $1128$, $1176$, $1225$:
 * $1275 = \displaystyle \sum_{k \mathop = 1}^{50} k = \dfrac {50 \times \left({50 + 1}\right)} 2$