Gwd 11-20

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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`

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)