Exercise 2, § 7

Let $(x_i)$ be a family of real numbers all of which belong to the interval $A'$ (resp. $A''$) and which satisfies condition b) of Exercise 1. Show that every subfamily of $(x_i)$ is summable in $\overline{\mathbf{R}}$, and that the sum of the $x'$ is associative (cf. Chapter III, § 5, no. 3, Theorem 2).