There are two types of cell references in Excel formulas and functions: relative and absolute. This is important because relative and absolute references behave differently when copied and filled to other cells. Relative references change when a formula is copied to another cell. Absolute references, on the other hand, remain constant no matter where they are copied.
It is important to understand the differences between the two types of cell references so that you can make your formulas return the results you intend.
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