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Q3(vii):
Taking the set of natural numbers as the universal set, write down the complements of the following sets: (vii) $\{ x : x$ is a perfect cube\}
Solution :
Step 1: Formal Definition of the Universal Set and Given Set
Given the Universal Set $U$ is the set of all natural numbers:
$U = \mathbb{N} = \{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...\}$ [Per the standard definition of Natural Numbers in set theory].
Let $A$ be the set defined in the problem:
$A = \{x : x \text{ is a perfect cube}\}$
Listing the elements of $A$ based on $n^3$ where $n \in \mathbb{N}$:
$A = \{1^3, 2^3, 3^3, 4^3, ...\} = \{1, 8, 27, 64, ...\}$
Step 2: Defining the Complement of a Set
The complement of a set $A$, denoted by $A^c$ or $A'$, is defined as the set of all elements in the universal set $U$ that are not in $A$.
Mathematically: $A^c = \{x : x \in U \text{ and } x \notin A\}$ [Per the definition of the Complement of a Set].
Step 3: Deriving the Complement Set
To find $A^c$, we subtract the set of perfect cubes from the set of natural numbers:
$A^c = \mathbb{N} \setminus \{x : x = n^3, n \in \mathbb{N}\}$
This implies that $A^c$ contains all natural numbers except those that can be expressed as $n^3$ for some integer $n \ge 1$.
Visual Representation
Step 4: Expressing the Result in Set-Builder Notation
Using the logical condition $x \neq n^3$, we represent the complement set as follows:
$A^c = \{x : x \in \mathbb{N} \text{ and } x \neq n^3 \text{ for any } n \in \mathbb{N}\}$
| Category | Description |
|---|---|
| Universal Set ($U$) | $\{1, 2, 3, 4, 5, 6, 7, 8, ...\}$ |
| Given Set ($A$) | $\{1, 8, 27, 64, ...\}$ |
| Complement ($A^c$) | $\{2, 3, 4, 5, 6, 7, 9, 10, ...\}$ |
Final Solution: The complement of the set $\{x : x \text{ is a perfect cube}\}$ is $\{x : x \in \mathbb{N}, x \text{ is not a perfect cube}\}$.
More Questions from Class 11 Mathematics Sets EXERCISE 1.5
- Q1(i): Let $U = \{ 1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{ 1, 2, 3, 4\}$, $B = \{ 2, 4, 6, 8 \}$ and $C = \{ 3, 4, 5, 6 \}$. Find (i) A′
- Q1(ii): Let $U = \{ 1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{ 1, 2, 3, 4\}$, $B = \{ 2, 4, 6, 8 \}$ and $C = \{ 3, 4, 5, 6 \}$. Find (ii) B′
- Q1(iii): Let $U = \{ 1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{ 1, 2, 3, 4\}$, $B = \{ 2, 4, 6, 8 \}$ and $C = \{ 3, 4, 5, 6 \}$. Find (iii) $(A \cup C)′$
- Q1(iv): Let $U = \{ 1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{ 1, 2, 3, 4\}$, $B = \{ 2, 4, 6, 8 \}$ and $C = \{ 3, 4, 5, 6 \}$. Find (iv) $(A \cup B)′$
- Q1(v): Let $U = \{ 1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{ 1, 2, 3, 4\}$, $B = \{ 2, 4, 6, 8 \}$ and $C = \{ 3, 4, 5, 6 \}$. Find (v) $(A′)′$
- Q1(vi): Let $U = \{ 1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{ 1, 2, 3, 4\}$, $B = \{ 2, 4, 6, 8 \}$ and $C = \{ 3, 4, 5, 6 \}$. Find (vi) $(B – C)′$
- Q2(i): If $U = \{ a, b, c, d, e, f, g, h\}$, find the complements of the following sets : (i) $A = \{a, b, c\}$
- Q2(ii): If $U = \{ a, b, c, d, e, f, g, h\}$, find the complements of the following sets : (ii) $B = \{d, e, f, g\}$
- Q2(iii): If $U = \{ a, b, c, d, e, f, g, h\}$, find the complements of the following sets : (iii) $C = \{a, c, e, g\}$
- Q2(iv): If $U = \{ a, b, c, d, e, f, g, h\}$, find the complements of the following sets : (iv) $D = \{ f, g, h, a\}$
- Q3(i): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (i) $\{x : x$ is an even natural number\}
- Q3(ii): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (ii) $\{ x : x$ is an odd natural number \}
- Q3(iii): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (iii) $\{x : x$ is a positive multiple of 3\}
- Q3(iv): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (iv) $\{ x : x$ is a prime number \}
- Q3(ix): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (ix) $\{ x : 2x + 5 = 9\}$
- Q3(v): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (v) $\{x : x$ is a natural number divisible by 3 and 5\}
- Q3(vi): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (vi) $\{ x : x$ is a perfect square \}
- Q3(viii): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (viii) $\{ x : x + 5 = 8 \}$
- Q3(x): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (x) $\{ x : x \ge 7 \}$
- Q3(xi): Taking the set of natural numbers as the universal set, write down the complements of the following sets: (xi) $\{ x : x \in N$ and $2x + 1 > 10 \}$
- Q4(i): If $U = \{1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{2, 4, 6, 8\}$ and $B = \{ 2, 3, 5, 7\}$. Verify that (i) $(A \cup B)′ = A′ \cap B′$
- Q4(ii): If $U = \{1, 2, 3, 4, 5, 6, 7, 8, 9 \}$, $A = \{2, 4, 6, 8\}$ and $B = \{ 2, 3, 5, 7\}$. Verify that (ii) $(A \cap B)′ = A′ \cup B′$
- Q5(i): Draw appropriate Venn diagram for each of the following : (i) $(A \cup B)′$
- Q5(ii): Draw appropriate Venn diagram for each of the following : (ii) $A′ \cap B′$
- Q5(iii): Draw appropriate Venn diagram for each of the following : (iii) $(A \cap B)′$
- Q5(iv): Draw appropriate Venn diagram for each of the following : (iv) $A′ \cup B′$
- Q6: Let U be the set of all triangles in a plane. If A is the set of all triangles with at least one angle different from 60°, what is A′?
- Q7(i): Fill in the blanks to make each of the following a true statement : (i) $A \cup A′ = . . .$
- Q7(ii): Fill in the blanks to make each of the following a true statement : (ii) $\phi′ \cap A = . . .$
- Q7(iii): Fill in the blanks to make each of the following a true statement : (iii) $A \cap A′ = . . .$
- Q7(iv): Fill in the blanks to make each of the following a true statement : (iv) $U′ \cap A = . . .$
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