Hi Roxanne.
fuzzylite is free and open-source. You can find the method here:
https://github.com/fuzzylite/fuzzylite/blob/master/fuzzylite/fl/Operation.h
Cheers.
FuzzyLite
The FuzzyLite Libraries for Fuzzy Logic Control
Fuzzy Logic Control was never this easy, simple, intuitive, beautiful, exciting,
and more importantly, completely based on free and open source technologies.