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 Th18:
  cosh.0 = 1
proof
  cosh.0 = (exp_R.0+exp_R.(-0))/2 by SIN_COS2:def 3
    .= 1 by SIN_COS:51;
  hence thesis;
end;
