A radical is a mathematical symbol used to denote “roots” of a value. The most common radical is used to denote a square root. The typical method of inserting a radical is to hold down the Alt key as you type 251 on the numeric keypad. Release the Alt key, and the symbol appears.
Of course, the appearance of the radical (or even whether it appears at all) depends on the font used in the cell. The Alt+251 method works for most normal fonts, but some fonts may not include the radical symbol (in which case it won’t appear) or may have the symbol mapped to a different character in the font. In that case, the best way to insert the symbol is to use the Symbol dialog box to search through the desired font and find the radical.
You can also use the Windows Character Map program to find the radical, copy it to the Clipboard, and then paste it into Excel.
All of the methods described so far are great if the only thing you want in the cell is the radical. You can, however, format a cell so that the radical symbol is displayed just to the left of whatever value is in the cell. Perhaps the easiest way to apply this format to a cell is to use a macro, as shown here:
Sub Radical() ActiveCell.NumberFormat = ChrW(8730) & "General" End Sub
Select the cell you want to format, then run the macro. (You can see how this custom format is handled by Excel if you run the macro and then display the Format Cells dialog box.)