I know that to find the sum of an arithmetic series I'd use [math]S=\frac{n}{2}[2a+(n-1)d][/math] but what do I do if I want the product of an arithmetic series (all the values multiplied together nicely)?

Sorry, not sure what you mean by the question. Do you mean that for your arithmetic series [imath]\sum_n a_n[/imath] you'd want to find [imath]\prod_n a_n[/imath] or what?

I think that's it but I don't recognise the symbol you've used.

Like, what's the value of the the first [math]x[/math] terms of a sequence [math]a+(n-1)d[/math] when they're all multiplied together?

hope this helps... otherwise... no idea:

http://en.wikipedia.org/wiki/Arithmetic_series#Product

Wow thanks. Now all I need to do is to understand gamma functions and I'm set. lol.