Weird acos values ?

[Tinia]

Hi,

one of our student found that those two examples below don't produce same 
result :

1. -->k = 0.5 * 44100 / 343;
    -->k = k / 44100 * 343;
    -->acos(k/0.5)
     ans  =

    2.107D-08i

2. -->k = 0.5;
    -->acos(k/0.5)
     ans  =

    0.


Same happens under 4.1.2 and 5.0.3. Any hint ?

Thanks in advance,

regards,

Rob.