# Complex Multiplication Distributes over Addition

## Theorem

The operation of multiplication on the set of complex numbers $\C$ is distributive over the operation of addition.

$\forall z_1, z_2, z_3 \in \C:$
$z_1 \left({z_2 + z_3}\right) = z_1 z_2 + z_1 z_3$
$\left({z_2 + z_3}\right) z_1 = z_2 z_1 + z_3 z_1$

## Proof

From the definition of complex numbers, we define the following:

$z_1 := \left({x_1, y_1}\right)$
$z_2 := \left({x_2, y_2}\right)$
$z_3 := \left({x_3, y_3}\right)$

where $x_1, x_2, x_3, y_1, y_2, y_3 \in \R$.

Thus:

 $\displaystyle z_1 \left({z_2 + z_3}\right)$ $=$ $\displaystyle \left({x_1, y_1}\right) \left({\left({x_2, y_2}\right) + \left({x_3, y_3}\right)}\right)$ $\quad$ Definition 2 of Complex Number $\quad$ $\displaystyle$ $=$ $\displaystyle \left({x_1, y_1}\right) \left({x_2 + x_3, y_2 + y_3}\right)$ $\quad$ Definition of Complex Addition $\quad$ $\displaystyle$ $=$ $\displaystyle \left({x_1 \left({x_2 + x_3}\right) - y_1 \left({y_2 + y_3}\right), x_1 \left({y_2 + y_3}\right) + y_1 \left({x_2 + x_3}\right)}\right)$ $\quad$ Definition of Complex Multiplication $\quad$ $\displaystyle$ $=$ $\displaystyle \left({x_1 x_2 + x_1 x_3 - y_1 y_2 - y_1 y_3, x_1 y_2 + x_1 y_3 + y_1 x_2 + y_1 x_3}\right)$ $\quad$ Real Multiplication Distributes over Addition $\quad$ $\displaystyle$ $=$ $\displaystyle \left({\left({x_1 x_2 - y_1 y_2}\right) + \left({x_1 x_3 - y_1 y_3}\right), \left({x_1 y_2 + y_1 x_2}\right) + \left({x_1 y_3 + y_1 x_3}\right)}\right)$ $\quad$ Real Addition is Commutative $\quad$ $\displaystyle$ $=$ $\displaystyle \left({x_1 x_2 - y_1 y_2, x_1 y_2 + y_1 x_2}\right) + \left({x_1 x_3 - y_1 y_3, x_1 y_3 + y_1 x_3}\right)$ $\quad$ Definition of Complex Addition $\quad$ $\displaystyle$ $=$ $\displaystyle \left({x_1, y_1}\right) \left({x_2, y_2}\right) + \left({x_1, y_1}\right) \left({x_3, y_3}\right)$ $\quad$ Definition of Complex Multiplication $\quad$ $\displaystyle$ $=$ $\displaystyle z_1 z_2 + z_1 z_3$ $\quad$ Definition 2 of Complex Number $\quad$

The result $\left({z_2 + z_3}\right) z_1 = z_2 z_1 + z_3 z_1$ follows directly from the above, and the fact that Complex Multiplication is Commutative.

$\blacksquare$