I was playing around with these two functions...
=SUM((A1:A10)*(MOD(ROW(A1:A10),2)))
=SUMPRODUCT(A1:A10,MOD(ROW(A1:A10),2))

The first formula being an array function (you have to press
ctrl+shift+enter). With
both of these functions I have to use a specified range, I can't use the
whole colum A:A. Why is this?