reserve r,t for Real;
reserve i for Integer;
reserve k,n for Nat;
reserve p for Polynomial of F_Real;
reserve e for Element of F_Real;
reserve L for non empty ZeroStr;
reserve z,z0,z1,z2 for Element of L;

theorem Th11:
  cos(4*PI/3) = -1/2
  proof
    4*PI/3 = PI+PI/3;
    hence thesis by EUCLID10:14,SIN_COS:79;
  end;
