You have to try to determine the sequence, and you try two basic kind of sequences: aritymethic and geometric.
In aritmetic sequeces the relationships is that the difference between any adjacent terms is constant.
For example
´Number of term (n) Term, An
1 7
2 11
3 15
4 19
Then the relationship between adjacent terms is 19 - 15 = 4 = 15 -11 = 4 = 11 - 7 = 4.
You can find, then, a general expression that relates any term with its position.
It is An = 7 + (n-1)*4
In geometric sequences the relationship is found dividing two adjacent terms, because the ratio is constant.
For example:
Number of term Term
1 10
2 20
3 40
4 80
You can then find the relation as: 20/10 = 2 = 40/20 = 2 = 80/40 = 2.
In this case the general term is An = 10 * 2^ (n-1)
