75
The simplest way to deal with this is to use
scales which are multiples of the default
scales. For example by using 13 13x−≤≤ and
6.2 6.4y−≤≤ (scale factor 2). You can also
use the ‘Square’ option on the ZOOM menu.
This adjusts the y axis so that it is ‘square’
relative to the x axis chosen.
The second issue is caused by
the domain of the circle being
undefined for some values. The
screen on your calculator is
made up of small dots called
pixels and is 131 pixels wide
and 64 pixels high. As was mentioned earlier,
this means that each pixel is 0.1 apart on the x
and y axes. This can affect your graphs and it
becomes particularly obvious with circles
because the graph does not exist for the part
of the x axis outside the circle. The two screen
shots right are an example of two images of the same graph
22
9xy
= using
two slightly different scales. You can see that the second example has
missing pieces.
Let's look at the circle
22
9xy+= as an example. This circle only exists from
-3 to 3 on the x axis and is undefined outside this domain. In order to graph it
on the hp 39g+ you have to rearrange it into
two equations of
F1(X)= (9-X
2
) for the top half &
F2(X)= -
(9-X
2
) for the bottom half.
If you enter these equations and then graph them with the default axes then
you get a perfect circle. However, if you change the x axis from the default
setting to -6 to 6 and then re-PLOT then part of the circle disappears. This is
shown in the snapshot at the start of the question.
The reason for this is that when the calculator draws the graph it does so by
‘joining the dots’. For the default scale of -6.5 to 6.5 this is not a problem
since the edges of the circle at -3 and 3 fall on a pixel. This means that the
last segment of the graph plotted extends is from 2.9 to 3 and the circle
reaches right down to the x axis.
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