reserve f,f1,f2,g for PartFunc of REAL,REAL;
reserve A for non empty closed_interval Subset of REAL;
reserve p,r,x,x0 for Real;
reserve n for Element of NAT;
reserve Z for open Subset of REAL;

theorem Th15:
  arccos (sqrt 2 /2) = PI/4
proof
  PI/4 < PI/1 by XREAL_1:76;
  hence thesis by Th13,SIN_COS6:92;
end;
