2n!=2n x (2n-1) x (2n-2) x ...
I have a set of convergence problems that I know I should solve with a ratio test,
but in denominator I have either or or even and I am little bit shakey with expanding these factorials for cancellation. I am only familiar with 5! or n! but not with factorial of sums or factorials of sums of products of variables and integers.
Is there a rule or a formula how could I do so? Would someone please show me how is and why expanded.