🚀 go-pugleaf

I am trying to develop some ideas around contingency tables and entropy, 
and have come up with some measures which I think are quite useful,
but the problem is that I have no idea in general how to compute variances for
these measures. 

It seems that the only general rule for computing variances is for the sum or
difference of random variables (ie var(x+/-y) = var(x) + var(y)). What about 
products, reciprocals and other arbitrary combinations of random variables. 
I realise that this is non-trivial and that there is no simple answer as 
there is for sums and differences but there must be techniques to get a 
handle on this.

Could someone point me in the right direction, references etc for this problem.

thanks,

Luke