Hello Barry,
starting with node programming isn't easy, but you have started it already - congratulation and welcome to the world of nodes. The problems in your file, is that your variables (inputs are numarrays) missing the element indexing - Always keep in mind that you are working with arrays!!!. To access the first element you have to use xxx[0]. Have a look in the attached file. The arrays are the requirement for multiprocessing data, even if they were not needed in this case. the same goes for the output.

Have a nice day
Karsten

p.s.: The variable R isn't used in your formula - If you use netbeans for programming or something similar, such things will be highlighted.