Maybe you're looking for something like this........
=IF(A1=B1,"",A1+B1)
Which says in Excel formula language, "if cell A1 is equal to cell B1, then
leave the cell where this formula is located blank, otherwise, put in it the
sum of A1 and B1.........
or maybe,
=IF(A1="",B1*2, B1*3)
Which says, if Cell A1 is empty, then put in the cell where this formula is
the product of B1 times 2, otherwise put B1 times 3...
The two quotation marks with no space between them represent the "null"
condition or for display purposes, a blank or empty cell...........
Vaya con Dios,
Chuck, CABGx3