pow and rootn functions

hello

Does anybody use pow and rootn functions  ?
I try to do a simple example : pow(2.0f, 4.0f) but it does not work ..
It seems  that pow does not return a proper value and code finishs without any error message.. It just finishes as a proper end , but it does not, as the value of 2 ^ 4 is not calculated.
The same for rootn ...

They are going to make me crazy, because it is very hard and difficult check and debug such kind of error..
My guess is that I am doing something wrong, but I am not able to realize WHERE, as I just use the function as in the especification .

Thanks in advance

Oh!!

For some weird thing , I 'm able to do "pow" .. but I am not able to do rootn ..
I am trying to do a simple
rootn(81.0, 3) ;

Any comments or suggestions ?

thanks in advance