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 Th39:
  arctan 1 = PI/4 & arctan.1 = PI/4
proof
A1: arctan 1 = arctan tan.(PI/4) by SIN_COS:def 28;
  PI/4 < PI/2 by Lm8,XXREAL_1:4;
  hence thesis by A1,Th35;
end;
