1.

Let the universal set U = {x : x is the natural number less than 20}. If A = {0, 2, 7, 15, 19, 20} and B = {5, 7, 9, 11, 13, 17, 20}, then (A - B) ∩ U is?1. Subset of U, A, but not of B2. Subset of U but not of A or B3. Subset of U, A and B4. Subset of U, B, but not of A

Answer» Correct Answer - Option 1 : Subset of U, A, but not of B

Concept: 

Set theory:

  • A ∪ B means set of all the values in the set A and B.
  • ∩ B is the set of common elements of A and B.
  • Subset (⊂) is the set such that all the elements of the subset are in the set from which the subset is taken from.
  • Number of elements in set A = n(A)
  • n(A ∪ B) = n(A) + n(B) - n(A∩B) = n(A) + n(B - A) = n(B) + n(A - B)
  • A' = U - A, where U is a universal set given and A is any set
  • n(A') = n(U) - n(A)
  • A - B = set values of set A not in set B
  • n(A - B) = n(A) - n(A ∩ B)

 

Calculation:

Given: U = {x : x is the natural number less than 20}.

A = {0, 2, 7, 15, 19, 20} and B = {5, 7, 9, 11, 13, 17, 20}

U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19}

A = {0, 2, 7, 15, 19, 20}

B = {5, 7, 9, 11, 13, 17, 20}

(A - B) = {0, 2, 15, 19}

(A - B) ∩ U = {2, 15, 19}

It is very clearly visible (A - B) ∩ U is a subset of U and A



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