\[f(x) = \sqrt{x - 8} - \sqrt{8 - x}\]
\[\left\{ \begin{matrix} x - 8 \geq 0 \\ 8 - x \geq 0 \\ \end{matrix} \right.\ \text{\ \ \ \ \ }\left\{ \begin{matrix} x \geq 8 \\ x \leq 8 \\ \end{matrix} \right.\ ;\ \ x = 8\ \ \ \]
\[D(f) = 8;\]
\[\Longrightarrow E(f) = 0.\]