引用第2樓ballballfull於2006-11-20 03:28發表的“”:
14. the sum S of the first n terms of the sequence 1, 5, 9, 13, ..., is given by the formula S=n(2n-1).
If S is greater than 435,
a. find the range of values of n.
b. what is the smallest value of n?
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a. S=n(2n-1)>435
2n^2-n-435>0
(n-15) (2n+29)>0
n>15
b. the smallest value of n is 16