http://www.colorforth.com/pi.htm
[edited to correct blatant error. Thanks Florin]
Either that or pi is a symbol and we use symbolic algebra. Just convert to the result in floating point at the last minute.
Ironically my calculator quite happily manages rational form including PI as a constant:
http://h41111.www4.hp.com/calculators/uk/en/scientific/smart...
http://www.colorforth.com/pi.htm
[edited to correct blatant error. Thanks Florin]