f(x)=a2+x2x−b2+(d−x)2(d−x) =a2+x2x+b2+(x−d)2(x−d) $\begin{array}{l}
f^{\prime}(x)=\frac{\sqrt{a^{2}+x^{2}}-\frac{x(2 x)}{2 \sqrt{a^{2}+x^{2}}}}{\left(a^{2}+x^{2}\right)} \
=\frac{a^{2}+x^{2}-x^{2}}{\left(a^{2}+x^{2}\right)^{3 / 2}}+\frac{b^{2}+(x-d)^{2}-(x-d)^{2}}{\left(b^{2}+(x-d)^{2}\right)^{3 / 2}} \
+\frac{\sqrt{b^{2}+(x-d)^{2}}-\frac{(x-d) 2(x-d)}{2 \sqrt{b^{2}+(x-d)^{2}}}}{\left(a^{2}+(x-d)^{2}\right)} \
\Rightarrow f^{\prime}(x)>0, \square x \in R
\end{array}\Rightarrow f(x)isincreasingfunction.Hence,f(x)$ is increasing function