A∪B∪C represents the area covered by three circles of ABC; A∩B∩C means that three conditions are met. Pay attention to two points in practical problem solving:
① The figure does not show anything that does not conform to ABC.
(2) A ∩ B contains A∩B∩C and only satisfies A∩B=A∩B-A∩B∩C, and so on.
definition
If there are three things to count: A, B and C, then the sum of elements of A, B and C = A+B+C-both A and B-both A and C-both B and C+ both A, B and C. (A ∪ B ∪ C = A+B+C-A ∪.
Q: Briefly 1949 The Third Geneva Treaty on the Treatment of Prisone