Copy Formulas Down:Row ref. every 10 rows

Your request from above is not correct in the sense that your first jump is actually 9 rows instead of 10 because A1 to A10 is only 9 rows.
However you could do what you want to do by following these steps:

After you have input the =A10+B10 in the cell
Click and select 9 cells below it.
Now click on the bottom right hand corner to drag
and fill down as many rows as you need.
What you have now is a formula every 10th row with
9 blank cells in between.
Next Select the column (or the number of cells in
the range with the blank cells) and press the F5
key (Function keys on top row of keyboard) to
get the Goto popup screen, click Special to get to the Go To Special popup screen
Click blanks on the left and click OK
The blank cells will be highlighted.
From the Edit menu select Delete and select the
Shifts cells up option.

That's it.