$\displaystyle \frac{x^4}{1-xy}$

find derivative with respect to y.holding x constant.

$\displaystyle f_y=\frac{[(4x^3)(1-xy)]-[(x^4)(-x)]}{(1-xy)^2}$

$\displaystyle f_y= \frac{4x^3-4x^4y+x^5}{(1-xy)^2}$

Is that right or did i do that wrong?

Or is is suppose to be

$\displaystyle f_y=\frac{[(0)(1-xy)]-[(x^4)(-x)]}{(1-xy)^2}$

$\displaystyle f_y=\frac{x^5}{(1-xy)^2}$

???? Suggestions?