Numbers: Force a calculation My apologies for such a basic question, but I have searched for many hours to no avail.  I have a very simple spreadsheet with two columns of data.  In the third column, I write a formula such that cell C1 contains  =$A1+$B1.  This works fine; the summation takes place as expected.  But, when I drag down the cell's formula into the cells below, the formula copies as expected, but the calculation does not update.  In MS Excel, I know how to fix this (there is an option to change the calculation behavior in Excel).  Could someone kindly show me how to handle this basic issue in OSX Numbers?  Thanks much.
In order to move the formula you have to drag the yellow circle on the bottom of the cell: