There must be two arrows (one on each end), and they act as ellipses do, such as is the case here:
...,-2, -1, 0, 1, 2, ...
The arrows show that the negative numbers continue to one side, as do the positive numbers
continue to the other side.
There is no difference when graphing a line in algebra/geometry. The drawn line has an arrow
on each end to show that it extends in both directions.
There is the wrong notion of the placement of arrows on the number line given at this [these] links, for example:
http://www.purplemath.com/modules/negative.htm
http://www.purplemath.com/modules/plane3.htm
...,-2, -1, 0, 1, 2, ...
The arrows show that the negative numbers continue to one side, as do the positive numbers
continue to the other side.
There is no difference when graphing a line in algebra/geometry. The drawn line has an arrow
on each end to show that it extends in both directions.
There is the wrong notion of the placement of arrows on the number line given at this [these] links, for example:
http://www.purplemath.com/modules/negative.htm
http://www.purplemath.com/modules/plane3.htm