theorem Th66:
  cos is_differentiable_on REAL & diff(cos,th)=-sin.th
proof
A1: [#]REAL is open Subset of REAL & REAL c=dom cos by FUNCT_2:def 1;
 for r st r in REAL holds cos is_differentiable_in r by Th62;
  hence thesis by A1,Th62,FDIFF_1:9;
end;
