Yesterday, the best answer is: cot(x)x.  (Contributed by dnialh. Good Job!)
dnialh solved the puzzle at 20:10:40.649; length: 18
dnialh solved the puzzle at 20:10:50.482; length: 18
dnialh solved the puzzle at 20:11:19.483; length: 18
dnialh solved the puzzle at 20:11:35.162; length: 18
fire solved the puzzle at 20:11:58.062; length: 18
dnialh solved the puzzle at 20:12:32.423; length: 18
dnialh solved the puzzle at 20:15:10.583; length: 26
dnialh solved the puzzle at 20:15:18.427; length: 26
dnialh is so fast solved the puzzle at 20:17:48.603; length: 18