Equivalence relation

Correct answer: A. Reflexive, symmetric, transitive

Step 1: An equivalence relation is identified by three properties. Step 2: It must be reflexive, symmetric, and transitive together. Step 3: In exams, never decide by checking only one or two properties.

Correct answer: A. Because every ((a,a)) is in the relation

Step 1: A relation is reflexive if every element is related to itself. Step 2: The universal relation contains all pairs of (A\times A), so each ((a,a)) is present. Step 3: For a non-empty set, the universal relation is always reflexive.

Correct answer: A. It is not reflexive

Step 1: Reflexivity is necessary for an equivalence relation. Step 2: The empty relation has no self-pairs such as ((1,1)). Step 3: On a non-empty set, the empty relation is not an equivalence relation.

Correct answer: A. ((a,c)\in R)

Step 1: An equivalence relation is transitive. Step 2: From ((a,b)) and ((b,c)), we must get ((a,c)). Step 3: When the middle element matches, check transitivity.

Correct answer: A. Equivalence relation

Step 1: Since (a-a=0) is even, the relation is reflexive. Step 2: If (a-b) is even, then (b-a) is even, and even differences combine transitively. Step 3: Same parity is a standard equivalence relation.

Correct answer: A. (3)

Step 1: Modulo (3), possible remainders are (0,1,2). Step 2: Integers with the same remainder form one class. Step 3: Modulo (n) usually gives (n) equivalence classes.

Correct answer: A. Equivalence relation

Step 1: Every element equals itself, so reflexivity holds. Step 2: Equality is symmetric and transitive. Step 3: Equality is the simplest example of an equivalence relation.

Correct answer: A. Equivalence relation

Step 1: (A\times A) contains every ordered pair. Step 2: Self-pairs, reverse pairs, and transitive pairs are all included. Step 3: The universal relation is always an equivalence relation.

Correct answer: A. Reflexive

Step 1: A relation is reflexive when every element is related to itself. Step 2: Pairs like ((a,a)) show this property. Step 3: Check reflexivity first when testing equivalence.

Correct answer: A. Symmetric

Step 1: Symmetry means the reverse pair must also belong to the relation. Step 2: If ((a,b)) is present, ((b,a)) must be present. Step 3: In exams, look for reverse pairs quickly.

Correct answer: A. Transitive

Step 1: Transitivity connects two related pairs to form a third pair. Step 2: When the middle element (b) matches, ((a,c)) must be present. Step 3: Order matters in transitivity.

Correct answer: A. It is actually an equivalence relation

Step 1: All self-pairs are present, so reflexivity holds. Step 2: The pair ((1,2)) has its reverse, and transitivity is not violated. Step 3: Check each property carefully before rejecting a relation.

Correct answer: A. ((3,3)) is missing

Step 1: Reflexivity needs every element of (A) to be related to itself. Step 2: The pair ((3,3)) is missing here. Step 3: First check all self-pairs in equivalence questions.

Correct answer: A. Equivalence relation

Step 1: Every number has the same parity as itself, so it is reflexive. Step 2: Same parity is symmetric and transitive. Step 3: Relations based on a shared category often form equivalence relations.

Correct answer: A. Same remainder (\pmod{5})

Step 1: If (a-b) is divisible by (5), the two integers have the same remainder modulo (5). Step 2: So the relation is based on congruence modulo (5). Step 3: For divisibility relations, think in terms of remainder classes.

Correct answer: A. Equivalence relation

Step 1: Every student is in the same class as themselves. Step 2: If (a) is in the same class as (b), then (b) is in the same class as (a). Step 3: Shared-group relations are common examples of equivalence relations.

Correct answer: A. Equivalence relation

Step 1: Every person has the same birthday date as themselves. Step 2: Having the same date is symmetric and transitive. Step 3: Relations based on an identical feature usually form equivalence relations.

Correct answer: A. Distinct disjoint equivalence classes

Step 1: An equivalence relation groups similar elements together. Step 2: These groups are disjoint and together cover the whole set. Step 3: Remember that equivalence classes form a partition.

Correct answer: A. Set of all elements related to (a)

Step 1: An equivalence class collects all elements related to a chosen element. Step 2: If an element is related to (a), it belongs to ([a]). Step 3: To find a class, list all elements related to the given element.

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

Step 1: In the same parity relation, odd numbers form one class. Step 2: (1) is odd and (3) is also odd. Step 3: For small sets, separate even and odd numbers first.

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

Step 1: (2) is even. Step 2: In the set, (4) is the other even number, so it is related to (2). Step 3: Write only the elements related to the chosen element.

Correct answer: A. Equivalence relation

Step 1: Every object has the same color as itself. Step 2: Same color is symmetric and transitive. Step 3: A relation based on an identical attribute often gives an equivalence relation.

Correct answer: A. It is not symmetric

Step 1: The pair ((1,2)) is in the relation. Step 2: For symmetry, ((2,1)) should also be present, but it is missing. Step 3: One missing reverse pair is enough to fail symmetry.

Correct answer: A. It is not symmetric

Step 1: All self-pairs are present, so reflexivity is fine. Step 2: ((1,2)) is present but ((2,1)) is missing, so symmetry fails. Step 3: For equivalence, every non-self pair needs its reverse pair.

Correct answer: A. The relation containing only pairs of the form ((a,a))

Step 1: In the identity relation, each element is related only to itself. Step 2: So its pairs have the form ((a,a)). Step 3: The identity relation is a basic example of an equivalence relation.