theorem Th30:
  for p being Point of TOP-REAL 2 holds |.p.| = sqrt((p`1)^2+(p`2) ^2)
proof
  let p be Point of TOP-REAL 2;
  (|.p.|)^2= (p`1)^2+(p`2)^2 by Th29;
  hence thesis by SQUARE_1:22;
end;
