(related to Proposition: Existence of Real Zero (Neutral Element of Addition of Real Numbers))
Note that since the rational Cauchy sequence of zeros \((0)_{n\in\mathbb N}\) exists, it is also true that the real number \(0:=(0)_{n\in\mathbb N} + I\) exists. It remains to be shown that the real number \(0\) is neutral with respect to the addition of real numbers. Let \(x=(x_n)_{n\in\mathbb N}+I\) be a real number. Because the rational Cauchy sequence \((0)_{n\in\mathbb N}\) is neutral with respect to the addition of rational Cauchy sequences, it follows that
\[\begin{array}{ccll} x+0&=&((x_n)_{n\in\mathbb N}+I)+((0)_{n\in\mathbb N}+I)&\text{by definition of real numbers}\\ &=&((x_n)_{n\in\mathbb N}+(0)_{n\in\mathbb N})+I&\text{by definition of adding real numbers}\\ &=&(x_n)_{n\in\mathbb N}+I&\text{because }(0)_{n\in\mathbb N}\text{ is neutral element of addition of rational Cauchy sequences}\\ &=&x&\text{by definition of real numbers}\\ \end{array}\]
It remains to be shown that also the equation \(0+x=x\) holds for all \(x\in\mathbb R\). It follows immediately from the commutativity of adding real numbers.