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https://en.wikipedia.org/wiki/Summation
Summation - Wikipedia
Summation of a sequence of only one summand results in the summand itself. Summation of an empty sequence (a sequence with no elements), by convention, results in 0. Very often, the elements of a sequence are defined, through a regular pattern, as a function of their place in the sequence.
Use the SUM function in Excel to sum a range of cells, an entire column or non-contiguous cells. To create awesome SUM formulas, combine the SUM function with other Excel functions.