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 Th14:
  cos(3*PI/4) = -sqrt 2 /2
proof
  cos(3*PI/4) = cos(PI/2 + PI/4) .= -sqrt 2/2 by Th7,SIN_COS:79;
  hence thesis;
end;
