## Calculus: Early Transcendentals 8th Edition

$f_x=4x^3+5y^3$, $f_y=15xy^2$.
$f(x,y)=x^4+5xy^3$ To find $f_x$ we treat y as a constant and differentiate with respect to $x$. $f_x=4x^3+5y^3$ To find $f_y$ we treat x as a constant and differentiate with respect to $y$. $f_y=15xy^2$