If your text already contains a formula, for example "12+24*2", LibreOffice can calculate, and then paste the result of the formula in your document, without using the Formula Bar.
Select the formula in the text. The formula can only contain numbers and operators and cannot contain spaces.
Choose Tools - Calculate, or press Command Ctrl+Plus Sign (+).
Place the cursor where you want to insert the result of the formula, and then choose Command Ctrl+V.
The selected formula is replaced by the result.