一道GRE数学
问题描述：
一道GRE数学
In a list of numbers,the first number is 3,the second is 4,and each subsequent number is the sum of all the proceeding numbers in the list.If the 20th number in the list is x,what is the 25th number in the list?A 7x B 16x C 32x D 35x E 161x
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答
数列如下
3,4,7,14,28,54.x.
3,4后每一个是前面数的和
20th x
21th 2x
22th 4x
24th 8x
25th 16x
B朋友是不是少算了23？20=x ,21=2x,22=4x,23=8x,24=16x,25=32x 是这样的吧？多谢了，看来这种题还是得先找规律，不能直接按照数列来套对C20th x21th2x22th4x23th8x24th 16x25th 32x