After you’ve graphed your parametric equations on your TI-83 Plus graphing calculator, you can use Zoom commands to change the viewing window of your graph. Pressing [ZOOM] accesses the Zoom menu. On this menu you will see all the Zoom commands that are available for graphing functions.

The Zoom commands that are most useful when graphing parametric equations are

• ZoomFit: This command finds a viewing window for a specified portion of the graph.

• ZSquare: Because the calculator screen is rectangular instead of square, circles will look like ellipses if the viewing window isn’t properly set. ZSquare adjusts the settings in the Window editor so that circles look like circles. To use ZSquare, first graph your parametric equations and then press [ZOOM][5]. The graph is then redrawn in a viewing window that makes circles look like circles. The following image illustrates the effect that ZSquare has on the spiral x = tcos(t), y = tsin(t).

• Zbox: Use this command to define a new viewing window for a portion of your graph by enclosing it in a box. The box then becomes the new viewing window. To construct the box, follow these steps:

1. Press [ZOOM][1].

2. Define a corner of the box.

To do so, use the

keys to move the cursor to the spot where you want one corner of the box to be located, and then press [ENTER]. The calculator marks that corner of the box with a small square.

3. Construct the box.

To do so, press the

keys. As you press these keys, the calculator draws the sides of the box.

When you use ZBox, if you don’t like the size of the box, you can use any of the

keys to resize the box. If you don’t like the location of the corner you anchored in Step 2, press [CLEAR] and start over with Step 1.

4. When you’re finished drawing the box, press [ENTER] and the graph is redrawn in the viewing window specified by the box.

When you use ZBox, [ENTER] is pressed only two times. The first time you press it’s to anchor a corner of the zoom box. The next time you press [ENTER] is when you’re finished drawing the box and are ready to have the calculator redraw the graph.

• Zoom In and Zoom Out: After the graph is drawn, these commands allow you to zoom in on a portion of the graph or to zoom out from the graph. They work very much like a zoom lens on a camera. To use these commands, follow these steps:

1. Press [ZOOM][2] if you want to zoom in, or press [ZOOM][3] if you want to zoom out.

2. Use the

3. keys to move the cursor to the spot on the screen from which you want to zoom in or zoom out.

This spot becomes the center of the new viewing window.

4. Press [ENTER].

You can press [ENTER] again to zoom in closer or to zoom out one more time.

5. Press [CLEAR] when you’re finished zooming in or zooming out.

If you used a Zoom command to redraw a graph and then want to undo what that command did to the graph, press

to select ZPrevious from the Zoom Memory menu. ZPrevious redraws the graph as it appeared in the previous viewing window.