Python Math libraries - How to?

[aguirre.adolfo at gmail.com]

Hi, thak you. I get the following:
>
> > *** Error message *************
>
> > Traceback (most recent call last):
> >   File "C:/Python25/z - MIS PROGRAMAS/volumen-b.py", line 20,
> >   in
> > <module>
> >     main()
> >   File "C:/Python25/z - MIS PROGRAMAS/volumen-b.py", line 13,
> >   in main
> >     volume = 4/3*pi*r3
> > NameError: global name 'pi' is not defined