If you type '+1/2, you will see '+1/2.
You can type " +1/2" preceding the + with a space (double quotes here are only to show the space), but this will change alignment one space to the right.
Also can use ="+1/2"and will see +1/2 left aligned as text; but this is a formula (double quotes here are mandatory).
Use a text editor, type there +1/2, then copy and paste in the cell. As long as it doesn’t start with = it will show as text, while the cell format will remain as number standard. Also can do it with Writer, but will need to choose menu Edit-Paste Special-Paste Unformatted Text (same that Ctrl+Alt+Shift+V).
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