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 Th41:
  tan.0 = 0 & tan 0 =0
proof
A1: 0 in ].-PI/2,PI/2.[ by XXREAL_1:4;
  then tan.0 = 0/cos.0 by Th1,RFUNCT_1:def 1,SIN_COS:30
    .= 0;
  hence thesis by A1,Th13;
end;
