Marvin's solution will work for addition and most other math operations, but if you enter 8/2, Excel will throw you a curve by displaying a date, which will look something like 2-Aug (depending on your locale and date/time settings). Surprise!
It would be preferable to format the column in which you want to show the construct of the formulas as the Text data type. Doing so disables formulas by converting them to text, as well as converting any other data likewise. And this way you get to show the formula with the equal sign intact.
If you ever have any troubles with your formulas, this topic on Office.com should help you:
Why is my formula broken?
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