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;
