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(A) SumProduct( A1:A3,B1:B3) == A1*B1 + A2*B2 + A3*B3
Instead, what I’m after is
(B) SumProduct( A1:A3, Reverse(B1:B3)) == A1*B3 + A2*B2 + A3*B1
Is there a clean way to achieve this in excel 2003 / excel 2007 ? The natural ordering of these values is A1->A3 and B1->B3, so reversing the meanings of the cells is unsatisfactory; but creating a reversed copy of B1:B3 elsewhere in the worksheet seems clumsy.
I cannot see a solution that doesn’t involve (a) custom functions in VBA (or similar) or (b) an extra column with partial results.
If you don’t like column C becoming a (hidden) reverse list, would you accept column C becoming a list like: A1*B3, A2*B2, A3*B1, which could then be summed? It would be possible to use a similar formula to the one mentioned in @e.tadeu’s answer to obtain this (using OFFET and ROW functions.)