Here is an arithmetic sequece find the 20 th term Find the 20th term of this sequence. 
$1, 5, 4, 8, 7, 11, 10, 14.........$
it is like add 4 to the st term then subtract 1 
i need to find a recursive formula that can help me get the 20th term. Help please.
There are two arithmetic series in the given series. The numbers at odd position constitute the following series 

$1,4,7,10,\cdots$

The numbers at even position constitute the following series:

$5,8,11,14,\cdots$

Since $20$ is an even number, the $20^{th}$ term will be the $10^{th}$ term of the second series. And it is $32$.
In Mathematica:

FindSequenceFunction[{1, 5, 4, 8, 7, 11, 10, 14}, 20]


(* 32 *)

and the function derived is:

$\frac{1}{4} (-1)^n \left(3 (-1)^n (2 n+1)+5\right)$