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 Th3:
  cos(x + 2 * n* PI) = cos x
proof
  cos.x = cos.(x+2*PI*n) by SIN_COS6:10;
  hence thesis;
end;
