Set is an important concept in modern mathematics, and the idea of set has become the theoretical basis of modern mathematics, which is widely related to many contents of high school mathematics. All kinds of objects studied in middle school mathematics can be regarded as sets or elements in sets, and mathematical concepts can be clearly expressed in set language.

Example: The probability of at least one head-up when tossing three coins is (Question 2 (3) 13 1 on page 3 required by People's Education Edition B).

Analysis: "At least one positive upward" event contains the possibility of 1 upward, 2 upward and 3 upward. The positive question is more complicated to answer, so we can consider the probability of "no positive upward once", that is, "all negative" from its negative side.