To quickly copy and paste formulas into adjacent cells, use the fill handle . When you fill formulas down, relative references are used to ensure formulas adjust for each row (unless you include absolute references or mixed references before you fill the formula down).
Select the cell that has the formula you want to fill into adjacent cells.
Drag the fill handle down or to the right of the column you want to fill.
Keyboard shortcut: You can also press Ctrl+D to fill the formula down a cell in a column, or Ctrl+R to fill the formula to the right in a row.
To learn more about using formulas, see Overview of formulas.