释义 |
Compound InterestLet be the Principal (initial investment), be the annual compounded rate, the ``nominal rate,'' be the number of times Interest is compounded per year (i.e., the year is divided into ConversionPeriods), and be the number of years (the ``term''). The Interest rate per ConversionPeriod is then
 | (1) |
If interest is compounded times at an annual rate of (where, for example, 10% corresponds to ), then theeffective rate over the time (what an investor would earn if he did not redeposit his interest after each compounding)is
 | (2) |
The total amount of holdings after a time when interest is re-invested is then
 | (3) |
Note that even if interest is compounded continuously, the return is still finite since
 | (4) |
where e is the base of the Natural Logarithm.
The time required for a given Principal to double (assuming Conversion Period) is given by solving
 | (5) |
or
 | (6) |
where Ln is the Natural Logarithm. This function can be approximated by the so-called Rule of 72:
 | (7) |
See also e, Interest, Ln, Natural Logarithm, Principal, Rule of 72, Simple Interest References
Kellison, S. G. The Theory of Interest, 2nd ed. Burr Ridge, IL: Richard D. Irwin, pp. 14-16, 1991.Milanfar, P. ``A Persian Folk Method of Figuring Interest.'' Math. Mag. 69, 376, 1996.
|