theorem Th67:
  sin is_differentiable_on REAL & diff(sin,th)=cos.th
proof
A1: [#]REAL is open Subset of REAL & REAL c=dom sin by FUNCT_2:def 1;
 for r st r in REAL holds sin is_differentiable_in r by Th63;
  hence thesis by A1,Th63,FDIFF_1:9;
end;
