Real Addition is Associative

Theorem
The operation of addition on the set of real numbers $\R$ is associative:
 * $\forall x, y, z \in \R: x + \left({y + z}\right) = \left({x + y}\right) + z$

Proof
From the definition, the real numbers are the set of all equivalence classes $\left[\!\left[{\left \langle {x_n} \right \rangle}\right]\!\right]$ of Cauchy sequences of rational numbers.

Let $x = \left[\!\left[{\left \langle {x_n} \right \rangle}\right]\!\right], y = \left[\!\left[{\left \langle {y_n} \right \rangle}\right]\!\right], z = \left[\!\left[{\left \langle {z_n} \right \rangle}\right]\!\right]$, where $\left[\!\left[{\left \langle {x_n} \right \rangle}\right]\!\right]$, $\left[\!\left[{\left \langle {y_n} \right \rangle}\right]\!\right]$ and $\left[\!\left[{\left \langle {z_n} \right \rangle}\right]\!\right]$ are such equivalence classes.

From the definition of real addition, $x + y$ is defined as $\left[\!\left[{\left \langle {x_n} \right \rangle}\right]\!\right] + \left[\!\left[{\left \langle {y_n} \right \rangle}\right]\!\right] = \left[\!\left[{\left \langle {x_n + y_n} \right \rangle}\right]\!\right]$.

Thus we have: