Trend using the same formula as the fill handle does

[Roy G. Biv]

Say you have a series of data points in a row. If you highlight the cells
containing that data series, then grab the fill handle and drag it over empty
cells to the right, Excel will trend the series forward. But suppose I want
to create a formula that does the exact same thing. For the sake of example,
say cells A2 through A10 have values in them already, and I want to create a
formula that would fill A11, A12, and A13 in the same way that dragging the
fill handle would.

Thanks!