Chapter 2 Relations And Functions EXERCISE 2.1
EXERCISE 2.1
1. If $\left(\dfrac{x}{3}+1, y-\dfrac{2}{3}\right)=\left(\dfrac{5}{3}, \dfrac{1}{3}\right)$, find the values of $x$ and $y$.
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Answer :
It is given that $\left(\dfrac{x}{3}+1, y-\dfrac{2}{3}\right)=\left(\dfrac{5}{3}, \dfrac{1}{3}\right)$
Since the ordered pairs are equal, the corresponding elements will also be equal.
Therefore, $\dfrac{x}{3}+1=\dfrac{5}{3}$ and $y-\dfrac{2}{3}=\dfrac{1}{3}$.
$\Rightarrow \ \dfrac{x}{3}+1=\dfrac{5}{3}$
$\Rightarrow \dfrac{x}{3}=\dfrac{5}{3}-1 $ $ \ \Rightarrow y-\dfrac{2}{3}=\dfrac{1}{3}$
$\Rightarrow \dfrac{x}{3}=\dfrac{2}{3} \quad \Rightarrow \ y=\dfrac{1}{3}+\dfrac{2}{3}$
$\Rightarrow x=2 \ \quad \Rightarrow \ y=1$
$\therefore \ \ x=2$ and $y=1$
2. If the set $A$ has $3$ elements and the set $B=\{3,4,5\}$, then find the number of elements in $\left(A \times B\right)$.
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Answer :
It is given that set $A$ has $3$ elements and the elements of set $B$ are $3, 4,$ and $5 .$
Number of elements in set $B=3$
Number of elements in $\left(A \times B\right)$ $=($ Number of elements in $A) \times($ Number of elements in $B)$
$\hspace{4.4cm}=3 \times 3=9$
Thus, the number of elements in $\left(A \times B\right)$ is $9 .$
3. If $G=\{7,8\}$ and $H=\{5,4,2\}$, find $G \times H$ and $H \times G$.
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Answer :
$G=\{7,8\}$ and $H=\{5,4,2\}$
We know that the Cartesian product $P \times Q$ of two non-empty sets $P$ and $Q$ is defined as
$\quad \ P \times Q=\{\left(p, q\right): p \in P, q \in Q\}$
$\therefore \ G \times H=\{\left(7,5\right),\left(7,4\right),\left(7,2\right),\left(8,5\right),\left(8,4\right),\left(8,2\right)\}$
$\quad H \times G=\{\left(5,7\right),\left(5,8\right),\left(4,7\right),\left(4,8\right),\left(2,7\right),\left(2,8\right)\}$
4. State whether each of the following statements are true or false. If the statement is false, rewrite the given statement correctly.
(i): If $P=\{m, n\}$ and $Q=\{n, m\}$, then $P \times Q=\{\left(m, n\right),\left(n, m\right)\}$.
(ii): If $A$ and $B$ are non-empty sets, then $A \times B$ is a non-empty set of ordered pairs $\left(x, y\right)$ such that $x \in A$ and $y \in B$.
(iii): If $A=\{1,2\}, B=\{3,4\}$, then $A \times\left(B \cap \phi\right)=\phi$.
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Answer :
$(i)$ Given $P=\{m, n\}$ and $Q=\{n, m\}$ then
$ \qquad \mathrm{P} \times \mathrm{Q}=\{(\mathrm{m}, \mathrm{n}),(\mathrm{m}, \mathrm{~m}),(\mathrm{n}, \mathrm{n}),(\mathrm{n}, \mathrm{~m})\} $
So, given value of $P \times Q$ is incorrect.
Hence, the given statement $(i)$ is false.
(ii) If $A$ and $B$ are non-empty sets, then $A \times B$ is a non-empty set of ordered pairs ( $x, y$ ) such that $x \in A$ and $y \in B$
Hence, the given statement $(ii)$ is true.
(iii) $A=\{1,2\}$ and $B=\{3,4\}$
$\qquad A \times(B \cap \phi)=A \times \phi=\phi$
So, $(iii)$ is true.
5. If $A=\{-1,1\}$, find $A \times A \times A$.
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Answer :
It is known that for any non-empty set $A, A \times A \times A$ is defined as
$\begin{aligned} &A=\{-1,1\}\\ &A \times A=\{-1,1\} \times\{-1,1\}=\{(-1,-1),(-1,1),(1,-1),(1,1)\}\\ &\Rightarrow A \times A \times A=\{-1,1\} \times\{(-1,-1),(-1,1),(1,-1),(1,1)\} \end{aligned}$
$\therefore ~ A \times A \times A=\{\left(-1,-1,-1\right),\left(-1,-1,1\right),\left(-1,1,-1\right),\left(-1,1,1\right), \left(1,-1,-1\right),\left(1,-1,1\right),\left(1,1,-1\right),\left(1,1,1\right)\}$
6. If $A \times B=\{\left(a, x\right),\left(a, y\right),\left(b, x\right),\left(b, y\right)\}$. Find $A$ and $B$.
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Answer :
It is given that $A \times B=\{\left(a, x\right),\left(a, y\right),\left(b, x\right),\left(b, y\right)\}$
We know that the Cartesian product of two non-empty sets $P$ and $Q$ is defined as $P \times Q=\{\left(p, q\right): p \in P, q \in Q\}$
$\therefore \ A$ is the set of all first elements and $B$ is the set of all second elements.
Thus, $A=\{a, b\}$ and $B=\{x, y\}$
7. Let $A=\{1,2\}, B=\{1,2,3,4\}, C=\{5,6\}$ and $D=\{5,6,7,8\}$. Verify that
(i): $ \ A \times\left(B \cap C\right)=\left(A \times B\right) \cap\left(A \times C\right)$
(ii): $ \ A \times C$ is a subset of $B \times D$
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Answer :
(i) To verify: $A \times\left(B \cap C\right)=\left(A \times B\right) \cap\left(A \times C\right)$
We have $B \cap C=\{1,2,3,4\} \cap\{5,6\}=\Phi$
$\therefore \ \ \text{L.H.S.} \ =A \times\left(B \cap C\right)=A \times \Phi=\Phi$
$A \times B=\{\left(1,1\right),\left(1,2\right),\left(1,3\right),\left(1,4\right),\left(2,1\right),\left(2,2\right),\left(2,3\right),\left(2,4\right)\}$
$A \times C=\{\left(1,5\right),\left(1,6\right),\left(2,5\right),\left(2,6\right)\}$
$\therefore \ \ \text{R.H.S.} \ \ =\left(A \times B\right) \cap\left(A \times C\right)=\Phi$
$\therefore \ \ \mathrm{L.H.S. = R.H.S.}$
Hence, $A \times\left(B \cap C\right)=\left(A \times B\right) \cap\left(A \times C\right)$
(ii): To verify: $A \times C$ is a subset of $B \times D$
$A \times C=\{\left(1,5\right),\left(1,6\right),\left(2,5\right),\left(2,6\right)\}$
$B \times D=\{\left(1,5\right),\left(1,6\right),\left(1,7\right),\left(1,8\right),\left(2,5\right),\left(2,6\right),\left(2,7\right),\left(2,8\right),\left(3,5\right),\left(3,6\right),\left(3,7\right),\left(3,8\right),\left(4,5\right),\left(4,6\right),\left(4,7\right),\left(4,8\right)\}$
We can observe that all the elements of set $A \times C$ are the elements of set $B \times D$
Therefore, $A \times C$ is a subset of $B \times D$.
8. Let $A=\{1,2\}$ and $B=\{3,4\}$. Write $A \times B$. How many subsets will $A \times B$ have? List them.
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Answer :
Given, $\mathrm{A}=\{1,2\}$ and $\mathrm{B}=\{3,4\}$
Now,
$ A \times B=\{(1,3),(1,4),(2,3),(2,4)\} $
Since $A \times B$ contains 4 elements, so number of subsets of $A \times B$ is $2^4=16$.
Therefore, the set $A \times B$ has $2^{4}=16$ subsets. These are
$\Phi,\{\left(1,3\right)\},\{\left(1,4\right)\},\{\left(2,3\right)\},\{\left(2,4\right)\},\{\left(1,3\right),\left(1,4\right)\},\{\left(1,3\right),\left(2,3\right)\}$,
$\{\left(1,3\right),\left(2,4\right)\},\{\left(1,4\right),\left(2,3\right)\},\{\left(1,4\right),\left(2,4\right)\},\{\left(2,3\right),\left(2,4\right)\}$,
$\{\left(1,3\right),\left(1,4\right),\left(2,3\right)\},\{\left(1,3\right),\left(1,4\right),\left(2,4\right)\},\{\left(1,3\right),\left(2,3\right),\left(2,4\right)\}$,
$\{\left(1,4\right),\left(2,3\right),\left(2,4\right)\},\{\left(1,3\right),\left(1,4\right),\left(2,3\right),\left(2,4\right)\}$
9. Let $A$ and $B$ be two sets such that $n\left(A\right)=3$ and $n\left(B\right)=2$. If $\left(x, 1\right),\left(y, 2\right),\left(z, 1\right)$ are in $A \times B$, find $A$ and $B$, where $x, y$ and $z$ are distinct elements.
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Answer :
It is given that $n(A)=3$ and $n(B)=2$ and $\{(x, 1),(y, 2),(z, 1)\}$ are in $A \times B$
We know that $A=$ Set of first elements of the ordered pair elements of $A \times B$
$B=$ Set of second elements of ordered pair elements of $A \times B$
$\therefore \ \ \mathrm{x}, \mathrm{y}$ and $z$ are the elements of $A$ and $1$ and $2$ are the elements of $B$
Since $n(A)=3$ and $n(B)=2$
it is clear that $A=\{x, y, z\}$ and $B=\{1,2\}$
10. The Cartesian product $A \times A$ has $9$ elements among which are found $\left(-1,0\right)$ and $\left(0,1\right)$. Find the set $A$ and the remaining elements of $A \times A$.
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Answer :
$ \mathrm{n}(\mathrm{~A} \times \mathrm{A})=9 $
Set $A=\{-1,0,1\}$ Since $(-1,0)$ and $(0,1)$ are elements in $A \times A$ Remaining elements
$ \qquad \ \ =\{(-1,-1),(-1,1),(0,-1),(0,0),(1,-1),(1,0),(1,1)\} $