=AVERAGE(Firstcell:lastcell) i think
You can use the Insert Function button fx on the Formula bar to find an Excel function by typing a description of the formula at the Insert Function dialog box, in the Search for a function text box and then click Go.
It has no special name. It is just the plus sign.
Fx represents functions. It is on the Formula Bar and on the Insert Function Button. Clicking on it starts the process of allowing you to enter a function.
If you mean doing calculations, then it is a formula.
yes
If you mean in Excel, yes: you must always start a formula with an equal sign.
There are lots of ways. Go to the Insert Menu and choose Function. Click on the Fx just beside the formula bar, or on the Formulas ribbon.
Change the formula to use absolute references instead of relative references. Instead of =A2+B3, use =$A$2+$B$3.
A green arrow in the corner of an excel cell means that excel suspects soemthing may be wrong with the formula in a particular cell.
The Formula Bar shows those things. Included in it is what is specifically known as the Name Box, which is where the address is shown. The Insert Function button is beside that and then the Formula Box part, when you can see the contents of the cell.
In Excel an equation can be a formula. All formulas in Excel must start with the equals sign. That is what tells Excel it is a formula.
Since you put this question in the Excel category, I will respond with an Excel formula. Assuming you put your numbers in cells A1 through A100, the formula would be:=SUM(A1:A100)/100