I just read the proof of Hardy-Littlewood-Sobolev inequality abaout fractional integral operator. Then I found the following identity (but don't understand it)
\begin{equation} \int_{% %TCIMACRO{\U{211d} }% %BeginExpansion \mathbb{R} %EndExpansion ^{n}\backslash B\left( x,R\right) }\frac{1}{\left\vert x-y\right\vert ^{c(n-1)}}dy=\int_{S^{n-1}}\int_{R}^{\infty }\frac{1}{r^{^{c(n-1)}}}% r^{n-1}drd\sigma \end{equation} where B(x,R) denote n dimensional ball, centered on $x$ and radius $R$. I guess that identity is generalization of substitution formula. Could you help me to understand that identity?