\begin{align} \label{19} \label{19}r & =\sqrt{\left(x_{1}\right)^{2}+\left(x_{2}\right)^{2}+\left(x_{3}\right)^{2}},\notag \\ \theta & =\cos^{-1}\left(\frac{z}{r}\right),\notag \\ \varphi & \label{06}=\tan^{-1}\left(\frac{y}{x}\right).\\ \end{align}