Modulo Multiplication Distributes over Modulo Addition

Theorem

$\forall \eqclass x m, \eqclass y m, \eqclass z m \in \Z_m$:
$\eqclass x m \times_m \paren {\eqclass y m +_m \eqclass z m} = \paren {\eqclass x m \times_m \eqclass y m} +_m \paren {\eqclass x m \times_m \eqclass z m}$
$\paren {\eqclass x m +_m \eqclass y m} \times_m \eqclass z m = \paren {\eqclass x m \times_m \eqclass z m} +_m \paren {\eqclass y m \times_m \eqclass z m}$

where $\Z_m$ is the set of integers modulo $m$.

That is, $\forall x, y, z, m \in \Z$:

$x \paren {y + z} \equiv x y + x z \pmod m$
$\paren {x + y} z \equiv x z + y z \pmod m$

Proof

Follows directly from the definition of multiplication modulo $m$ and addition modulo $m$:

 $\ds \eqclass x m \times_m \paren {\eqclass y m +_m \eqclass z m}$ $=$ $\ds \eqclass x m \times_m \eqclass {y + z} m$ $\ds$ $=$ $\ds \eqclass {x \paren {y + z} } m$ $\ds$ $=$ $\ds \eqclass {\paren {x y} + \paren {x z} } m$ $\ds$ $=$ $\ds \eqclass {x y} m +_m \eqclass {x z} m$ $\ds$ $=$ $\ds \paren {\eqclass x m \times_m \eqclass y m} +_m \paren {\eqclass x m \times_m \eqclass z m}$

And the second is like it, namely this:

 $\ds \paren {\eqclass x m +_m \eqclass y m} \times_m \eqclass z m$ $=$ $\ds \eqclass {x + y} m \times_m \eqclass z m$ $\ds$ $=$ $\ds \eqclass {\paren {x + y} z} m$ $\ds$ $=$ $\ds \eqclass {\paren {x z} + \paren {y z} } m$ $\ds$ $=$ $\ds \eqclass {x z} m +_m \eqclass {y z} m$ $\ds$ $=$ $\ds \paren {\eqclass x m \times_m \eqclass z m} +_m \paren {\eqclass y m \times_m \eqclass z m}$

$\blacksquare$