theorem Th5:
  for x being real-valued FinSequence holds |.x.| = sqrt |(x,x)|
proof
  let x be real-valued FinSequence;
  |.x.| = sqrt |.x.|^2 by EUCLID:9,SQUARE_1:22
    .= sqrt |(x,x)| by Th4;
  hence thesis;
end;
