Personal opinion, for reference only. According to the given conditions, figures and figures, first find out the results of the first four items (usually four items), and then analyze them to find out the relationship between each item and the ordinal number of the item (such as the relationship between the first item and 1; The relationship between the second item and the second item and so on. ), and then determine the general formula, that is, the relationship between the nth item and n.