### strange behaviour when solving equations symbolically

I'm new to sage and wanted to get startet. However, i tried solving equations symbolically and found a strang behaviour i do not understand. Can anybody explain to me this behaviour?

```
$ sage
----------------------------------------------------------------------
| Sage Version 4.7.1, Release Date: 2011-08-11 |
| Type notebook() for the GUI, and license() for information. |
----------------------------------------------------------------------
sage: r, u, m, M = var('r, u, m, M')
sage: qe = (u == sqrt(r^2 + m^2) + sqrt(r^2 + M^2))
sage: qe
u == sqrt(m^2 + r^2) + sqrt(M^2 + r^2)
sage: print solve(qe, r)
[
sqrt(M^2 + r^2) == u - sqrt(m^2 + r^2)
]
sage:
```

Any suggestions?