I want to add 2 columns of data and output the sum into a third column, with a =sum(a1+b1). With many rows of data, I try dragging down the highlighted cell to Fill the rest of the output column with the sum function. However, the top number is propagated to the rest of the rows, even though the function in the cell appears to be correct (for example =sum(a3+b3)). Is this a bug? My data is not text, no leading spaces, etc. Thanks.
No, it is not a bug. If you get the result you expected from keyeing F9 you might want to go to Tools, Calculate and check AutoCalculate. As an aside =a1+b1 should be sufficient instead of =sum(a1+b1). 

