You can do it like this, select the sheet with the formulas, hit Ctl+A to
select everything, right-click then Format Cells, select Protection tab and
uncheck Locked. Then in the menu; Edit->Go To->Special and check the formulas
box and hit ok, all cells with formulas are now selected. Right-click one of
these cells, select Format Cells, go to the protection tab and select locked.
Now when you protect the sheet, only the cells with formulas are protected.