reserve f,f1,f2,g for PartFunc of REAL,REAL;
reserve A for non empty closed_interval Subset of REAL;
reserve p,r,x,x0 for Real;
reserve n for Element of NAT;
reserve Z for open Subset of REAL;

theorem Th26:
  -cos is_differentiable_on REAL & for x st x in REAL holds diff(- cos,x)=sin.x
proof
  dom (-cos) = [#]REAL by FUNCT_2:def 1;
  hence thesis by Th23,Th25,SIN_COS:67;
end;
