`2,24,720,40320,3628800`

Let's write the series as,

`2*1,4*3*2*1,6*5*4*3*2*1,8*7*6*5*4*3*2*1,10*9*8*7*6*5*4*3*2*1`

From the above pattern we can write the series as,

`2!,4!,6!,8!,10!`

So based on the above pattern, we can write the n'th term of the series as:

`a_n=(2n)!`