Explain why the function is differentiable at the given point.

$(x,y)=8xln(xy-7),(2,4)$

The partial derivatives are $f_n(x,y)=$ and $f_r(x,y)=,$ so $f_2(2,4)=$ and $f_r(2,4)=$ esntinubus functions for $$ jse $f$ is differentiable at $(2,4).$

Find the linearlation $(x,y|)$ of $|f(x,y)|$ at $(2,4).$

$u(x,y)=$