Formula_4719

\[\begin{gathered}d x=d x_{1} d x_{2} d x_{3}, \quad d y=d y_{1} d y_{2} d y_{3}, \\|x-y|=\sqrt{\left(x_{1}-y_{1}\right)^{2}+\left(x_{2}-y_{2}\right)^{2}+\left(x_{3}-y_{3}\right)^{2}} .\end{gathered}\]