red warning sign in a cell in Numbers
Adding two numbers in a spreadsheet results in a triangular warning sign in the cell instead of the added numbers. Only in one cell.
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What is the error message? A syntax error means there is a mistake in the formula such as a typo or a missing/misplaced parenthesis or an invalid cell reference. "The operator + expects a number..." means one of the cells you are adding is not numeric (i.e, might be text). A reference error means the formula refers to a cell that does not exist. There are other errors but those are common ones.