Answer
If $a$ is an integer other than 0, then
a) 1 divides $a$
b) $a$ divides 0
Work Step by Step
Let $a$ be an integer other than 0.
For proof of a),
consider that 1 divides $a$ if there is an integer $c$ such that $a=\frac{1}{c}$. There is indeed such a $c$, and it is the multiplicative inverse of $a$, in fact.
Now, to prove b).
$a$ divides 0 because $c=0$ is an integer s.t. $a\cdot c=0$.
