reserve x,x0, r, s, h for Real,

  n for Element of NAT,
  rr, y for set,
  Z for open Subset of REAL,

  f, f1, f2 for PartFunc of REAL,REAL;

theorem Th14:
  for x be Real st x in ].0,PI.[ holds cot.x = cot x
proof
  let x be Real;
  assume x in ].0,PI.[;
  then cot.x = (cos x)/(sin x) by Th2,RFUNCT_1:def 1
    .= cot x by SIN_COS4:def 2;
  hence thesis;
end;
