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标题: ask one jj [打印本页]

作者: cook 时间: 2002-10-12 13:44 标题: ask one jj

3．A1=2,a2=3,a3=5.以后没想为前面所有项的和，问a24 是a20的多少倍。（16）

How to solve this problem? THank!

作者: zhouxh 时间: 2002-10-12 13:59

设bn 为第n项

则 a24=a1+..+a20+b21+b22+b23
b21=2a20 b22=4a20 b23=8a20 a1+..+a19=a20

所以 a24=(1+1+2+4+8)=16*a20

作者: cook 时间: 2002-10-12 15:58

ok, got it. I misunderstood a sequence like this: 2, 3, 5 ,8, 13,

an is the sum of numbers before it. thanks! I should learn to control the stress.

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