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Choose Table  Insert Table, and insert a table with one column and more than one row into a text document.

Type a number in each cell of the column, but leave the last cell in the column empty.

Place the cursor in the last cell of the column, and then click the
icon on the .
The appears with the entry "=sum". 
Click in the first cell of the series you want to sum up, drag to the final cell, and then release.
LibreOffice inserts a formula for calculating the sum of the values in the current column. 
Press Enter, or click Apply in the Formula bar.
The sum of the values in the current column is entered in the cell.
If you enter a different number anywhere in the column, the sum is updated as soon as you click in the last column cell.
Similarly, you can also quickly calculate the sum of a row of numbers.