Cumulative summing: sum rows 1-12, then rows 1-24, 1-36, etc..
 

I'm working on an ammortization calculation sheet. I want to add up
the cumulative interest, etc. I therefore want to sum the first year
(rows 1-12) then sum the first and second years (rows 1-24), then sum
the first, second, and third years (rows 1-36), etc...

Any idea on a method for accomplishing this?

Thanks.