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
  -1 <= r & r <= 1 implies cot arctan r = 1/r
proof
  set x = arctan r;
  assume that
A1: -1 <= r and
A2: r <= 1;
A3: sin x /cos x = tan x by SIN_COS4:def 1
    .= r by A1,A2,Th51;
  cot x = cos x/sin x by SIN_COS4:def 2
    .= 1/r by A3,XCMPLX_1:57;
  hence thesis;
end;
