Gwd 11-20

heyii

    

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Q20:

    

In the sequence 1, 2, 4, 8, 16, 32, …, each
term after the first is twice the previous term.  What is the sum of the 16th, 17th, and 18th
terms in the sequence?

    

 

    

A.    2¹⁸

    

B.     3(2¹⁷)

    

C.     7(2¹⁶)

    

D.    3(2¹⁶)

    

E.     7(2¹⁵)

    

Answer:

    

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这个题怎么做？谢谢哦`

aircavalry

观察数列的规律，1＝2^0 2=2^1 4=2^2...所以，n=2^(n-1)

于是有n16=2^15, n17=2^16, n18=2^17,

n16+n17+n18=2^15+2^16+2^17=(1+2+4)2^15=7(2^15)

heyii

噢噢噢噢噢噢。。。汗。。。多年没摸数学，居然不记得还有通项公式这个东西了！
谢谢了~相当于帮我解决了一类题目