How to reset/slightly change a formula in Excel after n number of rows

Question

I have a column in Excel that needs to take the continuous sum of another column for 8 rows and then start again with a new continuous sum for the next 8 rows. In other words, my current formula is SUM(A$1:A1) which is then pasted down until SUM(A$1:A9), but after this I need it to turn into SUM(A$10:10) and go until SUM(A$10:18) and so on. Any ideas on how to do so other than manually?

Answer 1

Use INDEX and some math:

=SUM(INDEX(A:A,(INT((ROW(1:1)-1)/9)*9 +1)):A1)