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The TI-89 has a 3-D graphing mode. In addition to drawing the graphs, you can spin the figure left, right, up, down (using the arrow keys), at 4 different speeds (hold the directional arrow key for a few seconds to get the figure spinning, then press the + key to speed up and the - key to slow down), view it from the x or y or z axis (press x, y or z after it is drawn), toggle between a zoom-in view and a normal view (press the multiplication key twice) and return the graph to the original position at any time by pressing 0.
Type the example below, z1(x,y)=(x^3*y-y^3*x)/390. From F1:Tools, option 9:Format, turn the axes and labels on. Notice different graph styles (including wire frame, hidden surface, contour levels) are possible. Use Zoom Standard for the graph below. Now spin the graph to get a better view! Press ENTER to pause the graph and then ENTER again to restart the spin. When finished, press 0 to return to the start position.
Contour levels, with or without wire frame graph, are available. Below is the wire frame graph with contour levels and the graph of just the contour levels of z1(x,y)=0.2x^2+0.2y^2-4 in a Zoom Standard window.
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