theorem Th14:
  |(An-Bn,An-Cn)| = - |(An-Bn,Cn-An)|
  proof
    reconsider rA=An,rB=Bn,rC=Cn as Element of REAL n by EUCLID:22;
    |(rA-rB,rA-rC)| = |(rA-rB,-(rC-rA))| by RVSUM_1:35
                   .= - |(rA-rB,rC-rA)| by EUCLID_4:24;
    hence thesis;
  end;
