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 Th37:
  arctan (-1) = -PI/4 & arctan.(-1) = -PI/4
proof
  -PI/2 < -PI/4 by Lm7,XXREAL_1:4;
  then arctan (-1) = -PI/4 by Th17,Th35;
  hence thesis;
end;
