Applet: Stack of cross sections
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Inside the transparent ellipsoidal region $x^2/16+y^2+z^2/4 \le 1$ are a stack of cross sections that are cut perpendicular to the $x$-axis. If you view the $x$-axis as being vertical, then the cross sections are stacked on top of each other. You can drag the cyan point on the slider to change the number of cross sections.
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General information about three.js applets
The applet was made using three.js and requires Javascript as well as a browser that supports WebGL. For most three.js applets, you can drag with the mouse to rotate the view, drag with the right button to pan, and zoom in/out with the mouse wheel. Many applets contain points that you can drag to change values of variables.