Formula that will take a number in a cell, and then add that number to itself -1, until itself -1 = 0

Question

I need a formula that will take a number in a cell, and then add that number to itself -1, until itself -1 = 0. For example, if the number were 5, the formula would be:

5 + 4 + 3 + 2 + 1

I could do this pretty easily in a for loop, but I hoping to avoid messing with sheets scripting.

Answer 1

I figured it out. Assuming the number in question is 38 minus whatever is in cell B15, the following formula works perfectly:

=((38-$B15)*$B15) + ((38-$B15) * (38-$B15)/2) - (38-$B15)/2

Answer 2

=ArrayFormula(SUM(ROW(INDIRECT("1:"&B1))))

...assuming the start number is in B1.

Essentially, this creates a reference to a range with as many rows as your start number, then uses SUM to add the row number property of each row.