reserve n for Nat;
reserve i for Integer;
reserve r,s,t for Real;
reserve An,Bn,Cn,Dn for Point of TOP-REAL n;
reserve L1,L2 for Element of line_of_REAL n;
reserve A,B,C for Point of TOP-REAL 2;

theorem Th16:
  |(Bn-An,Cn-An)| = - |(Bn-An,An-Cn)|
  proof
    |(Bn-An,Cn-An)| = |(An-Bn,An-Cn)| by Th15;
    hence thesis by Th14;
  end;
