i = interest
n = number of Years
asf = the annual sinking fund that is required to recoup $1 in n years at the
sinking fund rate of accumulation.
a = the sinking fund interest (or rate of accumulation)
Note: The above formulas are constructed on the assumption that interest is added at the end of the year.
Further Note: An annuity is an annual sum payable or receivable in equal amounts over a specified period.