I am trying to get Mathematica to give me the value of f'[x, r], where
f[x_, r_] := 4 r x (1 - x)
r = .5
But when I type, say, f'[.1, .5] I only get f'[.1, .5] back as the output. I do not understand why because it was giving me a number for the output just a few minutes before. How can I fix this?