Consider the following.

f$($x$,$ y$)=$

xy

$,$ P$(2,1),$ u $=$

$35$

$$i $+$

$45$

$$j

$($a$)$

Find the gradient of f$.$

$$f$($x$,$ y$)=$

$($b$)$

Evaluate the gradient at the point P$.$

$$f$(2,1)=$

$($c$)$

Find the rate of change of f at P in the direction of the vector u$.$

Duf$(2,1)=$