AMC 8 2009 Problem 5
AMC8 2009 年第 5 题
数列与规律★★☆☆☆
A sequence of numbers starts with 1 , 2 , and 3 . The fourth number of the sequence is the sum of the previous three numbers in the sequence: 1+2+3=6 . In the same way, every number after the fourth is the sum of the previous three numbers. What is the eighth number in the sequence?
一个数列以 1、2、3 开头。第四项是前三项之和：1+2+3=6。同样地，从第四项起每一项都是它前面三项之和。那么这个数列的第 8 项是多少？
- A.11
- B.20
- C.37
- D.68
- E.99
正确答案 · Correct Answer
D
解析 · Solution
Solution 1
List them out, adding the three previous numbers to get the next number, $1,2,3,6,11,20,37,68$
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