Infinitesimal method: take any small segment of X, x+dx, and rotate it around Y axis to get a hollow cylinder, and cut it parallel to Y axis to get a cuboid: thickness dx, width f(x) and length 2πx (circumference of a circle), so dV=2πxf(x)dx.

The solid obtained by rotation is a solid with a frustum in the middle and X = 2 as the rotation axis. The so-called [x, x+dx] between cells on [0, 1] is actually a circle with a thickness of dx at x, which surrounds a straight line X = 2. The circumference of a circle is 2π(2-x), and its height corresponds to the upper half of the circle.

The most common "exchange" techniques are as follows

(1) the mutual substitution of "time element" and "space element" (the most common in sports problems expressing the relationship between time and space);

(2) the mutual substitution of "volume element", "surface element" and "line element" (in essence, dimension reduction);

(3) the mutual substitution of "line element" and "angle element" (the transformation of the expression form of "element");

(4) Mutual substitution of "isolated elements" and "combined elements" (making full use of "symmetry" characteristics).