Well, formulas in general don't always start with a sign, but it might be talking about Excel, where they always start with an equal sign (=).
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No. SUM is a function. All formulas begin with the equals sign: =A3+B3 =SUM(A2:A25)
All formulas must begin with the equals sign. That is how Excel knows it is a formula.
You must start all formulas with an equal (=) sign. This symbol will tell Excel that you want to write a flormula.
All formulas must begin with the equals sign. You type that in and then follow with the rest of the formula using whatever parts you need to do what you need, like cell references, values, functions etc.
Cells in Excel worksheets can store many things, one of those being formulas. Formula entry is the act of putting formulas into cells. All formulas must begin with the equals sign. What happens after that will depend on what you want to do. See the related question below.
The equals sign goes at the beginning of a formula, as all formulas must begin with one. It is also possible to have the equals sign at other parts of a formula too.
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consistent definitions
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All formulas begin with the equals sign, so that is one symbol. A variety of other symbols can be used. Four other commonly used ones are the plus, minus, multiplication and division symbols.