This interactive Geogebra illustration demonstrates the idea of approximating the volume of a solid of revolution by the sum of volumes of thin disks (washers).[br][br]You can rotate the 3D view as follows. Place the mouse cursor anywhere in the 3D part of the illustrations. Press and hold the mouse and then drag to rotate the view. [br][br]You can experiment with different solids by entering your own expressions for f(x) and g(x) in the provided input fields. The parameters a, b are the end-points of the interval. When you enter your own expressions for [math]f(x)[/math] and [math]g(x)[/math], make sure that [math]g(x)\le f(x)[/math] for all [math]x\in\left[a,b\right][/math].[br][br]You can restore the original state of the illustration by clicking the two half-arrows in the upper right corner of the user interface part of the illustration.