reserve n for Nat,
  a, b, r, w for Real,
  x, y, z for Point of TOP-REAL n,
  e for Point of Euclid n;

theorem Th3:
  for f being real-valued FinSequence holds |.f.|^2 = Sum sqr f
proof
  let f be real-valued FinSequence;
  Sum sqr f >= 0 by RVSUM_1:86;
  hence thesis by SQUARE_1:def 2;
end;
