2. If the counted objects are divided into three categories: A, B and C, then:? The total number of counted objects = Class A elements+Class B elements+Class C elements, that is, the number of elements belonging to both Class A and Class B, Class A and Class C, Class B and Class C+Class A, Class B and Class C. ..
Extended data:
For example:
There are 48 people in Class 3 (1), among whom 32 people subscribe to Junior Newspaper, 38 to Math Newspaper and 25 to Two Newspapers. So what? How many people only subscribe to Youth Daily and not Math Daily?
Answer:
First draw a schematic diagram of newspaper subscription, as shown below: use a rectangular area to represent the number of students in the class; The ellipse with the letter A indicates that 32 people subscribe to Youth Daily; The ellipse with the letter B indicates that 38 people have subscribed to the Journal of Mathematics; The area where the letter C is located, that is, the overlapping part of two ellipses, indicates that 25 people have subscribed to two newspapers at the same time; The blank part where the letter D is located indicates the number of people who have not subscribed to either newspaper.
Yes, subtract the overlapping part C from the total number of people who subscribe to Youth Daily, and the number of people who only subscribe to Youth Daily but not to Math Daily is 32-25=7 (people).