Find the linear approximation $to$ the equation $f(x,y)=2\sqrt[\phantom{2}]{\frac{xy}{2}}at$ the point $(2,4,4),$ and use

$itto$ approximate $f(2\mathrm{.}15,4\mathrm{.}13)$

$f(2\mathrm{.}15,4\mathrm{.}13)$~$=$

Make sure your answer $is$ accurate $toat$ least three decimal places, $or$ give $an$ exact answer.