Maths · Number. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A set is a collection of things, called elements, written inside curly brackets. The universal set ξ holds everything being considered in the question. n(A) means the number of elements in A.
Two sets are combined in two ways. The intersection A ∩ B holds the elements that are in both sets. The union A ∪ B holds the elements that are in A or in B or in both. The complement A′ holds everything in ξ that is not in A.
A Venn diagram shows these regions as overlapping circles inside a rectangle. In counting problems, fill in the overlap first and then work outwards.
Adding n(A) and n(B) counts the overlap twice. Always subtract n(A ∩ B) once when you find the union.
Answer: 6
Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.