In our study and life, we often meet many numbers arranged in a certain order. Mathematically, we call such a set of numbers "series". Finding a rule of filling numbers is to observe the sequence first, then make strict logical reasoning, then find out the arrangement rule of numbers in the sequence, and fill in the missing numbers according to this rule, thus solving the problem.

For a series of numbers arranged in a certain order, we can know all the other numbers as long as we find the law from several consecutive numbers. For relatively simple series, the arrangement rules are generally found from the sum, difference, product and quotient of two adjacent numbers; A slightly complicated sequence should be analyzed reasonably and divided into several parts, and their arrangement rules should be observed respectively. Finding the law is the key to solve this kind of problem.

3、5、8、 13()()55

Because 3+5=8, 5+8= 13.

So: 8+ 13=(2 1),

13+2 1=(34).

2 1+34=55