Find the linear approximation $of$ the function $f(x,y,z)=\sqrt[\phantom{2}]{x^2+y^2+z^2}at(2,9,6)$ and use $itto$ approximate the number $\sqrt[\phantom{2}]{{2\mathrm{.}03}^2+{8\mathrm{.}98}^2+{5\mathrm{.}99}^2}.(Round$ your answer $to$ four decimal places.$)$

$f(2\mathrm{.}03,8\mathrm{.}98,5\mathrm{.}99)$~~