Copy a formula
You can copy any formula across a range of cells using your mouse. Once it’s entered, click on the cell, then click on the little box that appears in the lower right-hand corner. Drag it across the desired range of cells, and you will populate the range using the same formula.
This copies the formula across the cells, but changes the values relative to the new cell’s position. For example, in the B3 cell, the formula will read “=SUM(JAN!B3+FEB!B3)” without quotes. In the C3 cell, the formula will read “=SUM(JAN!C3+FEB!C3)” and so on.
The same thing can be done if you drag the box downward instead of across. This copies the formula into the column. So the B3 column formula will read “=SUM(JAN!B3+FEB!B3)” and the B4 formula will read “=SUM(JAN!B4+FEB!B4)” and so on.