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
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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
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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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
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
选C,理由如下。
The proceeding number in the list are as follows.
1. 3
2. 4
3. 3+4=7
4. 7+7=14
5. 7+7+7*2=7*4=28
6. 7+7+7*2+7*4=7*8=56
.......
Above a regulation could be ...
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选C,理由如下。
The proceeding number in the list are as follows.
1. 3
2. 4
3. 3+4=7
4. 7+7=14
5. 7+7+7*2=7*4=28
6. 7+7+7*2+7*4=7*8=56
.......
Above a regulation could be find that the n th number equals to 7*(2^(n-3))
So the 20th number is 7*(2^17).
The 25th number is 7*(2^22), which is 2^5=32 times as much as the 20th number.
Choose C.
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1.先看一下前6个数吧,分别是3,4,7,14,28,56...所以在第3为以后,你会发现下一个数就是上一个数的2倍;
2.知道这个规律后,来看一下第20到25个数:x,2x,4x,8x,16x,32x所以你看到了,第25个数就是32x,就是答案C。
希望对你有帮助啊!!!
题目翻译:在一个数列中,第一个数字为3,第2个数字为4,以后每个数字都输它前面数字的综合,如果数列中第20个数是X,那么第25个数是多少?
思路:不必管X具体是多少,只需要关注题目中“each subsequent number is the sum of all the proceeding numbers in the list”
把前19个数的和看做一个整体
因为第2...
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题目翻译:在一个数列中,第一个数字为3,第2个数字为4,以后每个数字都输它前面数字的综合,如果数列中第20个数是X,那么第25个数是多少?
思路:不必管X具体是多少,只需要关注题目中“each subsequent number is the sum of all the proceeding numbers in the list”
把前19个数的和看做一个整体
因为第20个数是\x09X,所以,前19个数的和也为X,以此类推
则第21个数为X+X=2X(前20个数的和为2X)
则第22个数为2X+2X=4X(前21个数的和为4X)
则第23个数为4X+4X=8X(前22个数的和为8X)
则第24个数为8X+8X=16X(前23个数的和为16X)
则第25个数为16X+16X=32X
所以答案选C
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