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 Th16:
  arccos (-sqrt 2 /2) = 3*PI/4
proof
  (3/4)*PI < 1*PI by XREAL_1:68;
  hence thesis by Th14,SIN_COS6:92;
end;
