4: Довжина

$\newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} }$ $\newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}}$$\newcommand{\id}{\mathrm{id}}$ $\newcommand{\Span}{\mathrm{span}}$ $\newcommand{\kernel}{\mathrm{null}\,}$ $\newcommand{\range}{\mathrm{range}\,}$ $\newcommand{\RealPart}{\mathrm{Re}}$ $\newcommand{\ImaginaryPart}{\mathrm{Im}}$ $\newcommand{\Argument}{\mathrm{Arg}}$ $\newcommand{\norm}[1]{\| #1 \|}$ $\newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $\newcommand{\Span}{\mathrm{span}}$ $\newcommand{\id}{\mathrm{id}}$ $\newcommand{\Span}{\mathrm{span}}$ $\newcommand{\kernel}{\mathrm{null}\,}$ $\newcommand{\range}{\mathrm{range}\,}$ $\newcommand{\RealPart}{\mathrm{Re}}$ $\newcommand{\ImaginaryPart}{\mathrm{Im}}$ $\newcommand{\Argument}{\mathrm{Arg}}$ $\newcommand{\norm}[1]{\| #1 \|}$ $\newcommand{\inner}[2]{\langle #1, #2 \rangle}$ $\newcommand{\Span}{\mathrm{span}}$