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 Th9:
  sin(11*PI/6) = -1/2
  proof
    11*PI/6 = 2*PI-PI/6;
    hence thesis by EUCLID10:3,17;
  end;
