A1 is the address of the first cell.
Cell protection only kicks in when you protect the entire sheet. So for the cell to be protected, then the sheet has to be protected.
By looking on the cell address
to select cells
To refer to another cell on another sheet, you precede the cell with the sheet name and an exclamation mark. So if you were on Sheet2 and wanted to refer to cell C6 on Sheet3 and add it to cell B7 on Sheet1 the formula would be: =Sheet3!C6 + Sheet1!B7
Ctrl-Home will bring you to cell A1 in a worksheet in Excel.
labels or cell content
relative cell address
the column in Ms excel are 16,384 and 1048,576 rows and 17,179,869 cell in one spread sheet. The spread sheet ends with the letter XFD.
This question is not clear. You can enter a number in Excel by clicking on a cell and typing the number from the keyboard.
Relative Address
Use the name of a sheet, followed by an exclamation mark and a cell reference. The following would take the value in cell C21 on Sheet2 and put it wherever the formula is typed. =Sheet2!C21
No it is not invalid. It is the cell address of the cell where column B meets row 17. It is a relative cell address.