A <spreadsheet tabular model> assigns a consistent meaning to each row and column. For an original example, let rows 2–4 describe three items, with quantity in column `A`, unit price in column `B`, subtotal in column `C`, and total including tax in column `D`. Put the single shared tax rate in `F1`. The chosen rate $1/5$ is an illustrative assumption. Quantities, unit prices and `F1` are <inputs>; columns `C` and `D` hold <spreadsheet formulas> and display their computed values as <outputs>.

Enter decimal <inputs> `2.5`, `1.75`, `4.5` for the unit prices and `0.2` for the tax rate. The following table expresses those values as exact <fractions> so that the <arithmetic> is easy to check. It does not require typing a fraction into an input <spreadsheet cell> or assume that all <software> calculates with exact <rational numbers>.

|| Row
|| Quantity (`A`)
|| Unit price (`B`)
|| Subtotal (`C`)
|| Total including tax (`D`)

| 2
| $3$
| $5/2$
| $15/2$
| $9$

| 3
| $4$
| $7/4$
| $7$
| $42/5$

| 4
| $2$
| $9/2$
| $9$
| $54/5$

Start with `C2` containing `=A2*B2` and `D2` containing `=C2*(1+$F$1)`. Here `*` requests <multiplication>, and `1+$F$1` adds the tax fraction to one. Copy these two <spreadsheet formulas> down into rows 3 and 4, a fill operation. The <relative spreadsheet references> change with the destination row: `C3` becomes `=A3*B3` and `D3` becomes `=C3*(1+$F$1)`; `C4` becomes `=A4*B4` and `D4` becomes `=C4*(1+$F$1)`. The <absolute spreadsheet reference> `$F$1` continues to read the shared rate. Copying <spreadsheet formulas> adjusts relative coordinates while retaining absolute coordinates. <Addresses and References, Absolute and Relative, LibreOffice Help>[Addresses and References, Absolute and Relative: Relative Addressing; Absolute Addressing; When to Use Relative and Absolute References, final two paragraphs]{ref}

\Image[/spreadsheet-copy-fill-shared-rate.png]
{title=Original three-row <spreadsheet tabular model> before and after filling two <spreadsheet formulas>, with a shared <absolute spreadsheet reference>}

The same <mathematical expression> applies to every item. Writing $q_i$ for quantity, $p_i$ for unit price and $t$ for the shared tax rate gives
$$
s_i=q_i p_i,\qquad d_i=s_i(1+t).
$$
For row 2, the exact <arithmetic> is $(3\cdot5/2)(1+1/5)=(15/2)(6/5)=9$. The subtotals add to $47/2$, so the totals add to $(47/2)(6/5)=141/5$, agreeing with $9+42/5+54/5$. The <spreadsheet formulas> encode the relationship; the displayed numbers are its results for these particular <inputs>.

This separation is useful for forecasting and <sensitivity analysis>. Keeping quantities and unit prices fixed while changing only `F1` from $1/5$ to $1/4$ gives exact totals $75/8$, $35/4$, and $45/4$. More generally, a rate change $\Delta t$ changes each total by
$$
d_i(t+\Delta t)-d_i(t)=s_i\Delta t.
$$
This follows by expanding the two <mathematical expressions> and cancelling $s_i(1+t)$. Here $\Delta t=1/20$, so the increases are $3/8$, $7/20$, and $9/20$, adding to $47/40$. One shared <input> makes the assumption visible and consistent across all rows. Overwriting a computed total with a number would instead remove that <spreadsheet formula>; its value would cease to follow the model when the rate changes.

// Target: computer-science.bigb


 Spreadsheet tabular model (source code)