✔ 最佳答案
Please read:
圖片參考:
https://s.yimg.com/rk/HA00430218/o/167123865.png
2014-07-22 16:28:38 補充:
First Step:
Consider the valuation ”at time 0” for:
"C at time 1" → C/(1 + r)
"C at time 2" → C/(1 + r)²
"C at time 3" → C/(1 + r)³
.
.
.
"C at time t" → C/(1 + r)^t
Then, summing all these values gives the present value of the annuity at time 0.
2014-07-22 16:29:07 補充:
Second Step:
Not "infinite" sum of geometric series.
This is just "finite" sum (有限項之和) of geometric series.
The number of terms is t in this case.
Sum of n consecutive terms in a geometric sequence:
a + ar + ... + ar^(n-1) = a(1 - r^n)/(1 - r)
2014-07-22 17:21:16 補充:
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令 S 為等比數列 n 項之和,其中 首項 = a,公比 = r
S = a + ar + ar² + ar³ + ... + arⁿ⁻² + arⁿ⁻¹ (共 n 項)
rS = ar + ar² + ar³ + ... + arⁿ⁻² + arⁿ⁻¹ + arⁿ
兩式相減:
S - rS = a - arⁿ
(1 - r)S = a(1 - rⁿ)
S = a(1 - rⁿ)/(1 - r)
2014-07-23 15:10:12 補充:
你可否說明一下你不明白的部分?
其實以上的是該公式的證明。
等比級數有限項和無限項之和是你高中時學過的,所以現在只是一個直接應用。
如果你可以準確地指出你不明白的部分,那才可以跟進解釋。