Example (3) Take the cars produced by Jinxing Automobile Factory in 2003 as an element and the cars produced in 2003 as a set.

Example (4) takes the countries that established diplomatic relations with China before June 65438+1 October12004 as the element.

Example (5) takes all squares as elements.

Example (6) takes the point whose distance to the straight line L is equal to the fixed length d as the element.

Example (7), Equation X? Take the real root of 3x-2=0 as the element.

Example (8) takes all senior one students enrolled in Xinhua Middle School in September 2004 as elements.

Set means that a certain class of substances with the same properties can be described in a unified way. The elements of a set contain uncertainties, which can be finite, such as examples (3), (4), (7) and (8), or infinite, such as examples (5) and (6). But as long as a certain feature can be described consistently, it can be a set.