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General Operating Instructions
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 In addition, the Σ+ key is used to compute the sum of the squares of all your entries, and keeps track of the number of entries you’ve made. (To see how
many entries you’ve made, simply press x y. But before you resume summing press x y again.)
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The mean is calculated by pressing ; the alternate function of this key ( Σ) permits you to continue summing—thereby modifying
the results of previous mean calculations.
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We’ve also provided an error recovery routine in case incorrect data is
entered. To correct your last entry calculation, after pressing Σ+, reenter the incorrect
value and press Σ+ (Σ–); you are back where you were before the incorrect entry. Note that a value must be entered prior to each Σ+ keystroke or errors
will result.
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Sum-of-Digits Calculations
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The sum-of-the-digits key SOD
is used to calculate an amortization schedule and related balances for
consumer finance charges or depreciation of capital assets.
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Amortization
on a period-by-period basis as well as the remaining balance after any
given period can be obtained. Furthermore, you have the option of
finding this information
at any point in the amortized life span and can obtain values for all
subsequent periods if you wish.
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Trend Line Calculations
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To forecast or estimate trends, you may chart a straight line of best fit through data points across a periodic (daily, weekly, etc.) time span. This type of analysis is
made possible through the TL (trend line)
key. The HP-80 calculates the course of a straight trend line by
operating on chronologically-entered data, and then projecting the line
to any point.
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