Sets and Relations

JEE Mathematics · 74 questions · Page 8 of 8 · Click an option or "Show Solution" to reveal answer

Q71
Let Z be the set of integers. If A = {x \in Z : 2(x + 2) (x2 - 5x + 6) = 1} and B = {x \in Z : - 3 < 2x - 1 < 9}, then the number of subsets of the set A × \times B, is
A 212
B 218
C 210
D 215
Correct Answer
Option D
Solution

A ={x

\in

z : 2(x+2)(x2 - 5x + 6) = 1} 2(x+2)(x2 - 5x + 6) = 20 \Rightarrow x = - 2, 2, 3 A = {-2, 2, 3} B = {x

ε\varepsilon

Z : - < 2x - 1 < 9} B = {0, 1, 2, 3, 4} A ×\times B has is 15 elements so number of subsets of A ×\times B is 215.

Q72
Let A = {0, 1, 2, 3, 4, 5}. Let R be a relation on A defined by (x, y) ∈ R if and only if max{x, y} ∈ {3, 4}. Then among the statements (S1): The number of elements in R is 18, and (S2): The relation R is symmetric but neither reflexive nor transitive
A both are false
B only (S1) is true
C only (S2) is true
D both are true
Correct Answer
Option C
Solution

To evaluate the relation R R on the set A={0,1,2,3,4,5} A = \{0, 1, 2, 3, 4, 5\} , we first need to understand the conditions for an element (x,y)(x, y) to be in R R .

Specifically, (x,y)R(x, y) \in R if and only if max{x,y}{3,4}\max\{x, y\} \in \{3, 4\}.

Considering this, let's list the pairs: For max{x,y}=3\max\{x, y\} = 3, the possible pairs are: (0,3),(3,0),(1,3),(3,1),(2,3),(3,2),(3,3)(0, 3), (3, 0), (1, 3), (3, 1), (2, 3), (3, 2), (3, 3) For max{x,y}=4\max\{x, y\} = 4, the possible pairs are: (0,4),(4,0),(1,4),(4,1),(2,4),(4,2),(3,4),(4,3),(4,4)(0, 4), (4, 0), (1, 4), (4, 1), (2, 4), (4, 2), (3, 4), (4, 3), (4, 4) Combining these, the set R R consists of the following elements: R={(0,3),(3,0),(1,3),(3,1),(2,3),(3,2),(3,3),(0,4),(4,0),(1,4),(4,1),(2,4),(4,2),(3,4),(4,3),(4,4)} R = \{(0, 3), (3, 0), (1, 3), (3, 1), (2, 3), (3, 2), (3, 3), (0, 4), (4, 0), (1, 4), (4, 1), (2, 4), (4, 2), (3, 4), (4, 3), (4, 4)\} This gives us a total of 16 elements in R R , not 18 as initially claimed in statement S1 S_1 .

Next, we analyze the properties of the relation R R : Reflexivity: A relation is reflexive if (x,x)R(x, x) \in R for all xA x \in A.

For example, (0,0),(1,1),(2,2)(0, 0), (1, 1), (2, 2) are not in R R , so R R is not reflexive.

Symmetry: A relation is symmetric if whenever (a,b)R(a, b) \in R, then (b,a)R(b, a) \in R as well.

For all pairs (x,y)(x, y) listed, both (x,y)(x, y) and (y,x)(y, x) are present.

Thus, R R is symmetric.

Transitivity: A relation is transitive if whenever (a,b)R(a, b) \in R and (b,c)R(b, c) \in R, then (a,c)R(a, c) \in R.

An example where transitivity fails is (0,3)(0, 3) and (3,1)(3, 1) are in R R but (0,1)(0, 1) is not in R R.

Therefore, R R is not transitive.

In conclusion, statement S2 S_2 is correct as R R is symmetric but neither reflexive nor transitive.

Q73
Let R be a relation from the set {1,2,3,,60}\{1,2,3, \ldots, 60\} to itself such that R={(a,b):b=pqR=\{(a, b): b=p q, where p,q3p, q \geqslant 3 are prime numbers}. Then, the number of elements in R is :
A 600
B 660
C 540
D 720
Correct Answer
Option B
Solution

We have a set S = {1, 2, 3, ..., 60}, and a relation R defined on the set S.

An element (a, b) belongs to the relation R if and only if b can be expressed as the product of two prime numbers p and q, where both p and q are greater than or equal to 3.

In terms of number theory, prime numbers are integers greater than 1 that have no divisors other than 1 and themselves.

We are interested in prime numbers that are greater than or equal to 3, because p and q must both be greater than or equal to 3.

The primes greater than or equal to 3 and less than or equal to 60 are {3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59}.

We need to find all possible values of b = p ×\times q such that b belongs to the set S.

We start by choosing the smallest prime number (which is 3) and keep multiplying it with all the prime numbers until the product exceeds 60 : 1.

If we start with p = 3, we can choose q to be 3, 5, 7, 11, 13, 17, or 19.

This gives us 7 valid products that are less than or equal to 60.

2.

If we start with p = 5, we can choose q to be 5, 7, or 11.

This gives us 3 valid products that are less than or equal to 60.

3.

If we start with p = 7, we can choose q to be 7.

This gives us 1 valid product that is less than or equal to 60.

So, we have a total of 7 + 3 + 1 = 11 possible values for b = p ×\times q that satisfy the conditions.

Since a can be any number in the set S, there are 60 possible values for a for each of the 11 values of b.

Therefore, the total number of elements in the relation R is 60 ×\times 11 = 660.

Q74
Let WW denote the words in the English dictionary. Define the relation RR by R={(x,y)W×WR=\{(x, y) \in W \times W \mid the words xx and yy have at least one letter in common}. Then, RR is
A reflexive, symmetric and not transitive
B reflexive, symmetric and transitive
C reflexive, not symmetric and transitive
D not reflexive, symmetric and transitive
Correct Answer
Option A
Solution

Let's evaluate the relation RR for the properties of reflexivity, symmetry, and transitivity.

Relation R : R=(x,y)W×WR={(x, y) \in W \times W \mid} the words xx and yy have at least one letter in common}.

Reflexivity : Each word in English obviously has at least one letter in common with itself, so the relation is reflexive.

Symmetry : If a word xx has at least one letter in common with a word yy, then yy necessarily has that same letter in common with xx.

So, the relation is symmetric.

Transitivity : This property is not always satisfied.

For instance, consider the three words 'cat', 'bat', and 'bee'.

'Cat' and 'bat' share a letter (the 'a'), and 'bat' and 'bee' share a letter (the 'b'), but 'cat' and 'bee' do not share any letters.

Therefore, even though 'cat' is related to 'bat' and 'bat' is related to 'bee', 'cat' is not related to 'bee', so the relation is not transitive.

In conclusion, the correct answer is Option A : RR is reflexive, symmetric and not transitive.

Ready for a full JEE mock test? Timed · full syllabus · instant results
Take a Mock Test →