F_out = [ F_in * (r + l) ]/ r

if i do differentiation to the respect of r, i get
-F_in * l / (r^2)

if i do differentiation to the respect of l, i get
F_in / r

Is that appropriate to say that further increase of r will decrease my F_out, but further increase of l will increase my F_out?

Reason of editing :: found a mistake in my second equation

Question

F_out = [ F_in * (r + l) ]/ r

if i do differentiation to the respect of r, i get 
-F_in * l / (r^2)

if i do differentiation to the respect of l, i get 
F_in / r

Is that appropriate to say that further increase of r will decrease my F_out, but further increase of l will increase my F_out?

Reason of editing :: found a mistake in my second equation

phi · Answer

further increase of r will decrease my F_out, but further increase of l will increase my F_out?

d F_out/dr = -F_in * l / (r^2)
says   F_out decreases (if I > 0),  for any value of  -infinity < r < + infinity


d F_out/dI  =  + F_in/r

that means  that if   r is positive,  then F_out increases, independent of how I changes