What does n(A ∩ B)= n(A)+n(B)-n(A ∪ B) mean?

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from a former college mathematics instructor ... First of all, taking precalculus is a waste of time. The most important thing to learn in mathematics before college is algebra. Take Algebra I and Algebra 2. Go to http://www.maa.org/pubs/octnov09pgs18-19.pdf and read letters by Agnes Rash, John Ward (JJWJ) and Mary Murphy. (Note: Most MAA FOCUS publications have 0 to 2 letters to the editor.) To read Joan Reinthaler's letter, go to http://www.maa.org/pubs/augsept09pgs32-33. Advice: If you plan to take calculus in college, try to spend a lot of time learning what you can in trigonometry and spend time practicing algebra. Second, the formula in your question has an intersection (upside down U) on the left and a union (U) on the right. n(C) means "the number of elements (people, single digits, days, whatever) in set C." Intersection of A and B = what is in both A and in B. Union of A and B = what is in either A or B (or both). Check that you understand this by making A = {1, 2, 4, 6} and B = {2, 3, 4, 5, 6, 7}.

from a former college mathematics instructor ... First of all, taking precalculus is a waste of time. The most important thing to learn in mathematics before college is algebra. Take Algebra I and Algebra 2. Go to http://www.maa.org/pubs/octnov09pgs18-19.pdf and read letters by Agnes Rash, John Ward (JJWJ) and Mary Murphy. (Note: Most MAA FOCUS publications have 0 to 2 letters to the editor.) To read Joan Reinthaler's letter, go to http://www.maa.org/pubs/augsept09pgs32-33. Advice: If you plan to take calculus in college, try to spend a lot of time learning what you can in trigonometry and spend time practicing algebra. Second, the formula in your question has an intersection (upside down U) on the left and a union (U) on the right. n(C) means "the number of elements (people, single digits, days, whatever) in set C." Intersection of A and B = what is in both A and in B. Union of A and B = what is in either A or B (or both). Check that you understand this by making A = {1, 2, 4, 6} and B = {2, 3, 4, 5, 6, 7}.