解:
设曲面方程上任一点为$(x_0,y_0,z_0)$,则在该点曲面的法向量为:
$\displaystyle \vec{n}=(F'_x,F'_y,F'_z)=(\frac{1}{2\sqrt{x_0}},\frac{1}{2\sqrt{y_0}},\frac{1}{2\sqrt{z_0}}),$
于是,切平面该点的方程为
$\displaystyle \frac{1}{2\sqrt{x_0}}(x-x_0)+\frac{1}{2\sqrt{y_0}}(y-y_0)+\frac{1}{2\sqrt{z_0}}(z-z_0)=0,$
即
$\displaystyle \frac{x}{\sqrt{x_0}}+\frac{y}{\sqrt{y_0}}+\frac{z}{\sqrt{z_0}}=\sqrt{a},$
切平面在三个坐标轴上的载距之和为:
$\displaystyle \sqrt{x_0}\sqrt{a}+\sqrt{y_0}\sqrt{a}+\sqrt{z_0}\sqrt{a}=\sqrt{a}(\sqrt{x_0}+\sqrt{y_0}+\sqrt{z_0})=a.$