Even thought I don't follow your calculations (A1+B1 = 1+5 = 10???), I think your are looking for the following:
Add your sheetname with a "!" before the cell in the formulabar, like this:
This can also easily be done by clicking the "=" next to the formulabar, choosing the SUM function, then select the cell on the first sheet, then select your second cell and the result in the formulabar will be the same.
Hope this is what you meant.