HP 39GS HP 39gs_40gs_Mastering The Graphing Calculator_English_E_F2224-90010.p - Page 68

The simplest way to deal with this is to use scales which are multiples of the default scales. For example by using −13 ≤ x ≤ 13 and −6.2 ≤ y ≤ 6.4 . These are a scale factor of 2 from the default axes of −6.5 ≤ x ≤ 6.5 and −3.1 ≤ y ≤ 3.2 . You can also use the Square option on the ZOOM menu. This adjusts the y axis so that it is 'square' relative to whatever x axis you have 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 x2 + y2 = 9 using two slightly different scales. You can see that the second example has missing pieces. Let's look at the circle x2 + y2 = 9 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 you have to rearrange it into two equations of: F1(X)= (9-X2) for the top half & F2(X)= - (9-X2) 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 to -6 to 6 rather that the default setting and then PLOT again you will find that part of the circle disappears. This is shown in the second snapshot above. 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 two half circles 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. 68