Symmetric relation

Correct answer: A. (8)

Step 1: The reflexive pairs ((1,1),(2,2),(3,3)) are compulsory. Step 2: The remaining off-diagonal reverse-pair groups are three. Step 3: Each group has two choices, so the total number is (2^3=8).

Correct answer: A. Symmetric

Step 1: To check symmetry from a matrix, compare entries across the main diagonal. Step 2: Here (m_{13}=m_{31}), (m_{12}=m_{21}), and (m_{23}=m_{32}). Step 3: If the relation matrix equals its transpose, the relation is symmetric.

Correct answer: A. (R) is symmetric but not reflexive

Step 1: If (|a-b|=1), then (|b-a|=1) also holds. Step 2: So every pair has its reverse pair, making the relation symmetric. Step 3: Since (|a-a|=0), the relation is not reflexive.

Correct answer: A. It is symmetric and reflexive

Step 1: ((a-b)^2=0) implies (a=b). Step 2: If (a=b), then (b=a), so symmetry holds. Step 3: For every (a), ((a-a)^2=0), so reflexivity also holds.

Correct answer: A. (R \cap S) is symmetric

Step 1: If ((a,b) \in R \cap S), then the pair lies in both (R) and (S). Step 2: Since both are symmetric, ((b,a)) lies in both. Step 3: Therefore, ((b,a) \in R \cap S), so intersection preserves symmetry.

Correct answer: A. ({(1,2),(2,1)})

Step 1: The option contains both ((1,2)) and ((2,1)), so it is symmetric. Step 2: But ((1,2)) and ((2,1)) would require ((1,1)) for transitivity, which is absent. Step 3: Symmetry and transitivity are different properties; reverse pairs do not automatically give transitivity.

Correct answer: A. (R^{-1}=R)

Step 1: In a symmetric relation, ((a,b) \in R) implies ((b,a) \in R). Step 2: The inverse relation (R^{-1}) contains exactly the reversed pairs. Step 3: Hence, for a symmetric relation, (R^{-1}=R).

Correct answer: A. Symmetric

Step 1: If (a-b) is divisible by (2), then (b-a=-(a-b)) is also divisible by (2). Step 2: Thus, each pair has its reverse pair. Step 3: In divisibility-based relations, a negative sign does not break symmetry.

Correct answer: A. The relation is not symmetric

Step 1: The matrix test for symmetry is (M=M^T). Step 2: If the matrix is not symmetric, then for some place (m_{ij}\neq m_{ji}). Step 3: That means some pair does not have its reverse pair, so the relation is not symmetric.

Correct answer: A. (T) will be symmetric

Step 1: Since (T\subseteq R), the pairs of (T) come from (R). Step 2: But (T) may include a pair while excluding its reverse pair, so symmetry can fail. Step 3: Symmetry is not always preserved when taking an arbitrary subset.

Correct answer: A. ({(1,2),(2,1),(2,3),(3,2)})

Step 1: The pairs ((1,2)) and ((2,3)) must be included. Step 2: Symmetry requires their reverse pairs ((2,1)) and ((3,2)). Step 3: The smallest symmetric relation contains only these necessary pairs.

Correct answer: A. ((y,x)\notin R)

Step 1: In a symmetric relation, if ((y,x)\in R), then ((x,y)\in R) must also be present. Step 2: The question states ((x,y)\notin R). Step 3: Therefore, ((y,x)\in R) would cause a contradiction, so ((y,x)\notin R).

Correct answer: A. It must be even

Step 1: Any off-diagonal pair ((a,b)) must appear with its reverse pair ((b,a)). Step 2: So off-diagonal pairs are counted in pairs. Step 3: Hence, their number must be even, even if the total number of relation pairs is odd.

Correct answer: A. (R) is symmetric

Step 1: If (a\cdot b) is even, then (b\cdot a) is the same product. Step 2: Hence, ((a,b)\in R) implies ((b,a)\in R). Step 3: For product-based relations, changing order does not change the product, which helps prove symmetry.

Correct answer: A. It is symmetric

The inverse relation R⁻¹ contains (b,a) whenever R contains (a,b). Consider any pair (a,b) in R∪R⁻¹. If it came from R, then (b,a) belongs to R⁻¹; if it came from R⁻¹, then (b,a) belongs to R. In both cases the reverse pair is in the same union, so R∪R⁻¹ is always symmetric. It need not be reflexive or transitive, and it is empty only in the special case R=∅.

Correct answer: A. (R)

Step 1: For a symmetric relation, (R^{-1}=R). Step 2: Hence, (R\cap R^{-1}=R\cap R). Step 3: The intersection of a set with itself is the same set, so the answer is (R).

Correct answer: A. (24)

Step 1: Exactly one of the three diagonal pairs must be chosen, giving (3) choices. Step 2: There are three off-diagonal reverse-pair groups, each with two choices. Step 3: Total number is (3\cdot 2^3=24).

Correct answer: A. Yes, because (a+b=b+a)

Step 1: If ((a,b)\in R), then (a+b=5). Step 2: Since (b+a=5) also holds, ((b,a)\in R). Step 3: In fixed-sum relations, changing the order does not change the condition, so symmetry holds.

Correct answer: A. ((3,1)) must be added

Step 1: The pair ((1,2)) has its reverse ((2,1)). Step 2: The pair ((2,3)) has its reverse ((3,2)). Step 3: The reverse of ((1,3)), namely ((3,1)), is missing, so it must be added.

Correct answer: A. If ((a,b)\in R), then ((b,a)\in R) must also be in (R)

Step 1: The main condition for a symmetric relation is that every pair must have its reverse pair. Step 2: In an antisymmetric relation, reverse pairs can occur together only when the two elements are equal. Step 3: In exams, separate symmetric and antisymmetric relations by their exact conditions, not by their names.

Correct answer: A. (64)

Step 1: For (4) elements, the number of independent symmetric choices is (\frac{4(4+1)}{2}=10). Step 2: The group containing ((1,2)) is forced to be included, and the group containing ((3,4)) is forced to be excluded, so two choices are fixed. Step 3: The remaining (8) choices give (2^8=256).

Correct answer: A. Symmetry

Step 1: ((1,2)) is paired with ((2,1)), and ((2,3)) is paired with ((3,2)). Step 2: Diagonal pairs are their own reverses, so they do not disturb symmetry. Step 3: Since ((3,3)) is missing, reflexivity fails, but symmetry definitely holds.

Correct answer: A. ( (4,2) )

Step 1: In a symmetric relation, if ((a,b) \in R), then ((b,a) \in R) must also be present. Step 2: ((1,3)) and ((3,1)) are paired, but ((2,4)) is missing its reverse ((4,2)). Step 3: In exams, always check the reverse of each unequal ordered pair.

Correct answer: A. 2⁶

For a set with n elements, a symmetric relation is determined by independent choices for all n diagonal pairs and for one member of each off-diagonal reverse pair. The number of such independent choices is n+n(n−1)/2=n(n+1)/2. Each chosen position may be included or omitted, giving 2^[n(n+1)/2] symmetric relations. With n=3, the exponent is 3(4)/2=6, so the answer is 2⁶. Option B counts all possible relations, not only symmetric ones.

Correct answer: A. It is symmetric

Step 1: The condition (a+b\text{ is even}) does not change when (a) and (b) are interchanged. Step 2: If ((a,b) \in R), then ((b,a) \in R) because (b+a=a+b). Step 3: For such rules, check whether swapping the positions changes the condition.