急!F6 Geometric Sequences 02q12

其實上次已經幾乎為你計完了，你可否寫寫你在那裏有困難呢？可能可以為你提供意見。

1. a)
i) 0.7 + 0.07 + 0.007 + .....
a ....... ar .........ar²

r = (ar) / (a) = 0.07/0.7 = 1/10

b) S(∞) = a/(1-r)
= (0.7)/[1-(1/10)]
= (0.7) / 0.9) = 7/9

2. Since r = 2

a ... ar .... ar² ..........
a ....a+20 ..........

ar = (a)(2) = a+20
a = 20

a .... ar .... ar² ..........
20 ...40 ... 80 ... 160 ............

Sn = a(1-r^n)/(1-r)
a) after 10 minutes, n = 10
S(10) = (20)(1-2^10)/(1-2)
= (20)(1-1024)/(-1)
= 20460

b) 1000000 = 20(1-2^n)/(1-2)
50000 = (1-2^n)/(-1)
-50000 = 1-2^n
2^n = 1+50000 = 50001
(n)log(2) = log(50001)
n = 15.6

after 16 minutes

2013-10-24 00:24:51 補充：
Sorry! 再睇番先知原來上次俾左D「山埃」你，冇心架，對唔住呀！

原來題目是增加了 20 ，所以應該是 a + 20 = ar = 20。

http://hk.knowledge.yahoo.com/question/question?qid=7013102200236