Bug or badly suboptimal result for derivative

[quinyu]

I'm also set to radians. Also, Exact, Use Root, no Princ, no Comp, no i, simplify=maximum. I'd expect it to work at least as good as simplify=minimum...

By the way, the HP 50g finds the proper derivative. So does the TI-nSpire CAS, the TI-89 Titanium, the TI-92, Mathematica 10.0, Maple 18, MATLAB R2015a (in MuPAD as well as in the main prog). Casio ClassPad II emu 2.0000.4000 does well with it too. So something is happening that's pretty specific to the prime.