Find $f_x$, $f_y$, $f_x(-5,1)$, and $f_y(4,-4)$ for the following equation.
$f(x,y) = \sqrt{x^2 + y^2}$
$f_x = $
(Type an exact answer, using radicals as needed.)
$f_y = $
(Type an exact answer, using radicals as needed.)
$f_x(-5,1) = $
(Type an exact answer, using radicals as needed.)
$f_y(4,-4) = $
(Type an exact answer, using radicals as needed.)