hello I need to do SUM(6-1) but don't know how to do it the result have to be 15
If you are talking about the sum of the first n-1 positive integers then it is given by:
SUM(n-1) = (n-1)*n/2
so SUM(6-1) = (6-1)*6/2 → 5*3 → 15
(i.e. 1 + 2 + 3 + 4 + 5 = 15)
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