Euclid's Lemma

Theorem
Let $a, b, c \in \Z$.

If $a \backslash b c$, where $a$ and $b$ are relatively prime, then $a \backslash c$.

Proof
$a \perp b$ from the definition of relatively prime.

That is, $\gcd \left\{{a, b}\right\} = 1$.

From Integer Combination of Coprime Integers, we may write $a x + b y = 1$ for some $x, y \in \Z$.

Upon multiplication by $c$, we see that $c = c \left({a x + b y}\right) = c a x + c b y$.

Since $a \backslash a c$ and $a \backslash b c$, it is clear that $a \backslash \left({c a x + c b y}\right)$.

However, $c a x + c b y = c \left({a x + b y}\right) = c \cdot 1 = c$.

Therefore, $a \backslash c$.

Also see

 * Euclid's Lemma for Prime Divisors