reserve X for set,
  Y for non empty set;
reserve n for Nat;
reserve r for Real,
  M for non empty MetrSpace;
reserve n for Nat,
  p,q,q1,q2 for Point of TOP-REAL 2,
  r,s1,s2,t1,t2 for Real,
  x,y for Point of Euclid 2;

theorem Th25:
  dist(q-q1,q-q2) = dist(q1,q2)
proof
  -(q-q1)= q1-q & -(q-q2) = q2-q by RLVECT_1:33;
  hence dist(q-q1,q-q2) = dist(q1-q,q2-q) by Th24
    .= dist(q1,q2) by Th23;
end;
