# Unexpected behaviour of arbitrary precision real numbers

I've been trying to use arbitrary precision real numbers and I'm a bit confused. The simple snippet below illustrates my point:

```
R = RealField(200)
z = R(2.0)
print '%.20f' % (z.sqrt())
```

I should get (or that's what I was expecting) 1.41421356237309504880, but instead I got 1.41421356237309514547 (a disagreement in the last five decimal places). Where am I goofing up? You see, it's a quite simple code. How could I fix it in order to obtain the desired/expected result?

Thanks in advance for any light shed on this matter.

Fausto