Set, Relations and FunctionsPYQ Nov. 18Question 1886 of 217
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If A=(1,2,3,4,5,6,7)\displaystyle A = (1, 2, 3, 4, 5, 6, 7) and B=(2,4,6,8)\displaystyle B = (2, 4, 6, 8). Cardinal number of A−B\displaystyle A - B is:

Options

A4
B3
C9
D7
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Correct Answer

✅ Option a — 4

All Options:

  • A4
  • B3
  • C9
  • D7

Detailed Solution & Explanation

We are given two sets:
A={1,2,3,4,5,6,7}\displaystyle A = \{1, 2, 3, 4, 5, 6, 7\}
B={2,4,6,8}\displaystyle B = \{2, 4, 6, 8\}
The difference set A−B\displaystyle A - B is defined as the set containing elements that belong to A\displaystyle A but do not belong to B\displaystyle B:
A−B={x∣x∈A and x∉B}A - B = \{x \mid x \in A \text{ and } x \notin B\}
Let us identify the common elements of A\displaystyle A and B\displaystyle B:
A∩B={2,4,6}A \cap B = \{2, 4, 6\}
Subtracting these common elements from A\displaystyle A:
A−B={1,2,3,4,5,6,7}−{2,4,6}={1,3,5,7}A - B = \{1, 2, 3, 4, 5, 6, 7\} - \{2, 4, 6\} = \{1, 3, 5, 7\}
The cardinal number (or cardinality) of a set is the number of elements in it. Let n(A−B)\displaystyle n(A - B) represent the cardinal number of A−B\displaystyle A - B:
n(A−B)=4n(A - B) = 4
Hence, **Option A** is the correct answer.

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