I have this formula in Cell A3:
sum(A1+A2)

I want to fill handle this cell and have the cell A4 as below:
sum(A1+A3)

As you see, the A1 must be fixed. But this operation makes the A4 as
below:
sum(A2+A3) which is not my aim.

You can use an absolute reference for A1...

=SUM(A$1+A2)

Rick