Question:

The number of subsets of $\{1, 2, 3, ..., 9\} $containing at least one odd number is

Updated On: Jun 17, 2022
  • 324
  • 396
  • 496
  • 512
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The Correct Option is C

Solution and Explanation

The total number of subsets of given set is $2^{9}=512$
Even numbers are $\{2,4,6,8\}$
Case I: When selecting only one even number $={ }^{4} C_{1}=4$
Case II: When selecting only two even numbers $={ }^{4} C_{2}=6$
Case III: When selecting only three even numbers $={ }^{4} C_{3}=4$
Case IV : When selecting only four even numbers $={ }^{4} C_{4}=1$
$\therefore$ Required number of ways
$=512-(4+6+4+1)-1=496$
[Here, we subtract 1 for due to the null set]
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Concepts Used:

Sets

Set is the collection of well defined objects. Sets are represented by capital letters, eg. A={}. Sets are composed of elements which could be numbers, letters, shapes, etc.

Example of set: Set of vowels A={a,e,i,o,u}

Representation of Sets

There are three basic notation or representation of sets are as follows:

Statement Form: The statement representation describes a statement to show what are the elements of a set.

  • For example, Set A is the list of the first five odd numbers.

Roster Form: The form in which elements are listed in set. Elements in the set is seperatrd by comma and enclosed within the curly braces.

  • For example represent the set of vowels in roster form.

A={a,e,i,o,u}

Set Builder Form: 

  1. The set builder representation has a certain rule or a statement that specifically describes the common feature of all the elements of a set.
  2. The set builder form uses a vertical bar in its representation, with a text describing the character of the elements of the set.
  3. For example, A = { k | k is an even number, k ≤ 20}. The statement says, all the elements of set A are even numbers that are less than or equal to 20.
  4. Sometimes a ":" is used in the place of the "|".