45

Number
$45$ (forty-five) is:


 * $3^2 \times 5$


 * The $9$th triangular number after $1, 3, 6, 10, 15, 21, 28, 36$:
 * $45 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = \dfrac {9 \times \left({9 + 1}\right)} 2$