The first term of a G.P. is 5 and the last term is 1280. If the sum of all the terms is 2555, find the number of terms. How to solve?
回答 (1)
2015-11-11 12:30 pm
✔ 最佳答案
a = 5a*R^(n-1) = 1280
R^(n-1) = 1280/a = 1280/5 = 256 ..... (1)
R^n = 256R
a( 1 - R^n ) / ( 1 - R ) = 2555
( 1 - 256R ) / ( 1 - R ) = 2555/a = 2555/5 = 511
1 - 256R = 511 - 511R
255R = 510
R = 2 , substitute into (1)
2^(n-1) = 256 = 2^8
n = 9 ..... Ans
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