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;
