theorem
  for x,y being real-valued FinSequence st len x=len y holds |.x+y.|^2 - |.x
  -y.|^2 = 4* |(x,y)|
proof
  let x,y be real-valued FinSequence;
  assume
A1: len x=len y;
  then |.x+y.|^2 - |.x-y.|^2 = (|.x.|^2 + 2*|(y,x)| + |.y.|^2) - |.x-y.|^2 by
Th10
    .= (|.x.|^2 + 2*|(x, y)| + |.y.|^2) - (|.x.|^2 - 2*|(y, x)| + |.y.|^2)
  by A1,Th11
    .= 4*|(x, y)|;
  hence thesis;
end;
