Bug or badly suboptimal result for derivative

[roadrunner]

That's weird. It only gives me the red X error when I do that assignment in Home. In CAS it works ok.

Sometimes I get strange random CAS variables that mess things up. I don't know what I do to get them but maybe you have some too. Check your CAS variables and reset any that look strange, also make sure you don't have f, g, u, x, etc. predefined as something strange.

-road