Returns the depreciation of an asset for a given year using the fixed rate declining-balance method.


DB(originalcost, salvagevalue, lifetime, year, months1styear)

originalcost: the initial cost of the asset.
salvagevalue: is the value at the end of the depreciation (sometimes called the salvage value of the asset).
lifetime: the number of years over which the asset is being depreciated.
year: the year number for which the depreciation is calculated.
months1styear: the number of months in the first year (defaults to 12 if omitted).
To calculate depreciation, DB uses a fixed rate throughout the asset’s life, given by
   rate = 1 - (salvagevalue/originalcost)(1/lifetime)
rounded to 3 decimal places.
The depreciation in any year is given by
   value_at_start_of_year * rate
where value_at_start_of_year = originalcost - depreciation_so_far.
If months1styear is less than 12 the depreciation rate used for the first and last years is
   rate * number_of_months_in_year / 12.


DB(10000, 1000, 5, 1)
returns approximately 3690 in currency units, which is the depreciation in the first year for an asset which cost 10000 and is written down to 1000 over 5 years.

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