Exercises
[11F] Let \((Z,\| \| _ Z)\) be an additional normed space, and \(B:Y→ Z\) a linear application. We similarly define
\[ \| B\| _{Y,Z}{\stackrel{.}{=}}\sup _{y∈ Y~ ,~ \| y\| _ Y≤ 1} \| B y \| _ Z\quad ; \]show that
\[ \| A B\| _{X,Z} ≤ \| A\| _{X,Y} \| B\| _{Y,Z}~ ~ . \]