reserve x,y for Real,
  u,v,w for set,
  r for positive Real;

theorem Th9:
  for a,b being Real holds |.|[a,b]|.|^2 = a^2+b^2
proof
  let a,b be Real;
A1: |[a,b]|`2 = b by EUCLID:52;
  |[a,b]|`1 = a by EUCLID:52;
  hence thesis by A1,JGRAPH_1:29;
end;
