The spreadsheet is a large rectangular grid of boxes into which you can insert numbers, messages or commands. This sounds excruciatingly dull, but it has an important property that makes it incredibly useful as a mathematical tool.
You can copy the contents of a box into others very easily; and when you do so the commands in it change to be the same relative to the new box as they were relative to the old.
Let us see what can be done with this.
Suppose we insert the number -3 in box A1, and put '=a1+1' in box A2 and also in box B1. (Don't put the marks '; they are here to enclose what to put in.)
If we now copy A2 into A3...A1000 (very simple to do when you get the hang of it) we will get what in column A?
Similarly copying B1 into C1,...,CD1 will produce the same result in row 1.