theorem Th28:
  cos`|REAL = -sin
proof
A1: for x being Element of REAL st x in dom (cos`|REAL) holds (cos`|REAL).x
  = (-sin).x
  proof
    let x be Element of REAL;
    assume x in dom (cos`|REAL);
    (cos`|REAL).x = diff(cos,x) by FDIFF_1:def 7,SIN_COS:67
      .= -sin.x by SIN_COS:63;
    hence thesis by VALUED_1:8;
  end;
  dom (-sin) = REAL by FUNCT_2:def 1;
  then dom (cos`|REAL) = dom (-sin) by FDIFF_1:def 7,SIN_COS:67;
  hence thesis by A1,PARTFUN1:5;
end;
