If you add the first two elements (3,1) and multiply the result by 2, you get the third element (8).
In general, to find any element from a3 onwards, you add the previous two and double the result.
As a recursive formula, it will be like:
[tex]a_{1} = 3, a_{2} = 1, a_{n}=a_{n-1} + a_{n-2}[/tex]
