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Theorem (Division Algorithm)
Let $a,n\in\mathbb{Z}$, with $n> 0$. Then there exists unique integers $q$ and $r$ (called the quotient and remainder) such that $a=qn+r$ where $0\leq r<n$.
Example
If we let $a=23$ and $n=5$, then we can write $23=4\cdot 5+3$. The quotient is $q=4$ and the remainder is $r=3$. Notice that $0\leq r<5$.
Non Example
Let $a=-23$ and $n=5$. We can write $-23=(-4)\cdot 5-3$, so it might be tempting to say that the quotient is $q=-4$ and the remainder is $r=-3$. However notice that it is not true that $0\leq r<5$. The remainder cannot be negative, and must be less than $n$. Instead we can write $-23=(-5)\cdot 5+2$. This means the quotient is $q=-5$ and the remainder is $r=2$.