Let f$($x$,$y$)=$xex$2$y and P$=(9,81).($a$)$ Calculate $$fP$($b$)$ Find the rate of change of f in the direction $$fP$.($c$)$ Find the rate of chang$$ of f in the direction of a vector making an angle of $45$ with $$fP$.$ Answers : $($a$)($b$)($c$)$