425

Number
$425$ (four hundred and twenty-five) is:


 * $5^2 \times 17$


 * The $17$th pentagonal number after $1$, $5$, $12$, $22$, $35$, $51$, $70$, $92$, $117$, $145$, $176$, $210$, $247$, $287$, $330$, $376$:
 * $425 = \displaystyle \sum_{k \mathop = 1}^{17} \left({3 k - 2}\right) = \dfrac {17 \left({3 \times 17 - 1}\right)} 2$