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View Full Version : A number line, or a line in general, has two arrows.



lookagain
01-06-2011, 12:28 AM
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

mmm4444bot
01-06-2011, 02:04 PM
they act as ellipses do

Ellipses is the plural of ellipse; I think that you mean ellipsis dots (), also known simply as "an ellipsis".


There is the wrong notion of the placement of arrows on the number line given at [purplemath]

The convention that I learned for the Real number line is that the arrow indicates the positive direction. This is the same convention that I use when drawing the vertical and horizontal Real number lines which comprise the xy-coordinate axes.

lookagain
01-06-2011, 07:29 PM
they act as ellipses do

Ellipses is the plural of ellipse; I think that you mean ellipsis dots (), also known simply as "an ellipsis".


> \ > "Ellipses" \ \ is \ also \ \ the \ \ spelling \ \ of \ \ the \ \ plural \ \ of \ \ "ellipsis," \ and \ \ was \ \ meaning \ \ (both \ \ of) \ \ the \ ellipses,\

one \ \ at \ \ each \ \ end \ \ of \ \ the \ \ visible \ \ partial \ \ list \ \ of \ \ integers \ \ there. \ < \ <


There is the wrong notion of the placement of arrows on the number line given at [purplemath]

The convention that I learned for the Real number line is that the arrow indicates the positive direction.
This is the same convention that I use when drawing the vertical and horizontal Real number lines which comprise the
xy-coordinate axes.


> \ > \ \ And \ \ the \ \ Real \ \ number \ \ line \ \ extends \ \ infinitely \ \ in \ \ two \ \ directions, \ \ as \ \ do \ \ the \ \

x-axis \ \ and \ \ the \ \ y-axis. \ \ \ And \ \ there \ \ is \ \ no \ \ inconsistency \ \ with \ \ the \ \ positions \ \ of \ \

the \ \ numbers; \ \ the \ \ larger \ \ numbers \ \ are \ \ to \ \ the \ \ right \ \ (or \ \ above \ \ for \ \ the \ \ upper \ \ y-axis),

\ \ and \ \ the \ \ smaller \ \ numbers \ \ are \ \ to \ \ the \ \ left \ \ (or \ \ below \ \ for \ \ the \ \ lower \ \ y-axis ). \ \ < \ <


The left-hand arrow on the Real number line points in the direction of decreasing numbers, such as in the
inequality 0 - 3 < 2, and the right-hand arrow on the Real number line points in the direction of
increasing numbers, such as in the inequality 6 + 5 > 9.

mmm4444bot
01-07-2011, 02:54 PM
Not all axes have positive to the left or up. Some are not even labeled with numbers. I like the convention where the arrow indicates the positive direction (left, right, up, or down). I would hope that people understand by context that a line extends forever in both directions, yet I feel that there is nothing wrong with putting arrows at each end of the segment drawn, just as there is nothing wrong with using one arrow. I prefer the latter. 8-)

garyk02453
10-06-2014, 12:51 PM
Though you are correct that infinite nature of lines that represent integer (Z) and Real (R) numbers in both directions needs to be mentioned, please point me at a single mathematical or physics diagram outside of elementary school that shows 2 arrows? 4 arrows on (XY)? May be 6 on (X,Y,Z)?

I.e.: http://users-phys.au.dk/philip/pictures/physicsfigures/node13.html

Positive direction of axis is shown on diagrams with an arrow. To show that line is not terminated, it's shown past the 0 or other mark. It is usually a bad practice to manipulate well accepted conventions...


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