t">\(a\)</span> with constant density <span class="math notranslate nohighlight">\(\rho_0\)</span>:</p>
<div class="math notranslate nohighlight">
\[\begin{split}D_r =\frac {4\pi} {r}\int\limits ^ r_0\rho (\xi)\xi d\xi =
\begin{equation*}
 \begin{cases}
   \displaystyle 2\pi\rho_0 r, r &lt;a,
    \\
   \displaystyle\frac {2\pi\rho_0 a ^ 2} {r}, r&gt; a.
 \end{cases}
\end{equation*}\end{split}\]</div>
<p>Given that in a vacuum <span class="math notranslate nohighlight">\(D = E\)</span>, <span clas