Equivalence relation

Correct answer: A. ({4,9})

Step 1: (9) gives remainder (4) on division by (5). Step 2: In the given set, (4) and (9) both have remainder (4). Step 3: In a modulo relation, the class is decided by the common remainder.

Correct answer: A. ((2,8))

Step 1: All elements in the same equivalence class are related to one another. Step 2: (2) and (8) both belong to ({2,5,8}). Step 3: Therefore ((2,8)) must be in the relation.

Correct answer: A. ([x]=[y])

Step 1: In an equivalence relation, related elements belong to the same class. Step 2: Since ((x,y)\in R), the classes of (x) and (y) are equal. Step 3: Do not treat equivalence classes of related elements as different.

Correct answer: A. (a\leq b)

Step 1: From (a\leq b) and (b\leq c), we get (a\leq c), so the relation is transitive. Step 2: But (2\leq 3) does not imply (3\leq 2), so symmetry fails. Step 3: Transitivity alone is not enough for equivalence.

Correct answer: A. Equivalence relation

Step 1: Every number has the same number of digits as itself. Step 2: If two numbers have the same number of digits, the reverse statement is also true. Step 3: This equality of digit count is transitive, so the relation is an equivalence relation.

Correct answer: A. All three-digit natural numbers

Step 1: (145) has three digits. Step 2: Therefore its class is the set of all three-digit natural numbers. Step 3: Apply the defining property directly to the chosen element.

Correct answer: A. Equivalence relation

Step 1: Every line is identical to itself, so reflexivity holds. Step 2: If (l) is parallel to (m), then (m) is parallel to (l). Step 3: Lines with the same direction preserve transitivity, so the relation is an equivalence relation.

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

Step 1: (2) is less than (4). Step 2: The elements less than (4) are (1,2,3), so the class of (2) is ({1,2,3}). Step 3: In grouping relations, first identify the group of the given element.

Correct answer: A. ({1,5},{2,4},{3})

Step 1: (1) and (5) are mutually related, so they form one class. Step 2: (2) and (4) form another class, while (3) is related only to itself. Step 3: Identify connected groups from the given ordered pairs.

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

Step 1: Reflexivity requires ((1,1),(2,2),(3,3)). Step 2: Only the first option contains all three diagonal pairs. Step 3: Check all diagonal pairs first when testing reflexivity.

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

Step 1: Symmetry requires ((b,a)) whenever ((a,b)) is present. Step 2: The first option has both ((1,2)) and ((2,1)), and ((3,3)) reverses to itself. Step 3: If the reverse of a non-diagonal pair is missing, symmetry fails.

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

Step 1: From ((1,2)) and ((2,3)), transitivity requires ((1,3)). Step 2: The first option contains this required pair. Step 3: In transitivity, use the middle element to identify the needed start-to-end pair.

Correct answer: A. ({1,2},{3},{4})

Step 1: The condition (a=b) relates every element to itself. Step 2: The extra condition relates (1) and (2) to each other. Step 3: Hence (1,2) form one class, while (3) and (4) remain singleton classes.

Correct answer: C. (9)

Step 1: A class of size (m) contributes (m^2) ordered pairs. Step 2: The total is (2^2+2^2+1^2=4+4+1). Step 3: Therefore the relation has (9) pairs.

Correct answer: A. (a) is related to every element of (A)

Step 1: ([a]) is the set of all elements related to (a). Step 2: ([a]=A) means every element of (A) is related to (a). Step 3: Use the definition of an equivalence class for such conclusions.

Correct answer: A. Because (aRa) is always true

Step 1: Reflexivity is required in an equivalence relation. Step 2: Reflexivity gives (aRa), so (a\in[a]). Step 3: Therefore no equivalence class is empty.

Correct answer: A. ({1,3,5},{2,4,6})

Step 1: Having the same remainder modulo (2) means having the same parity. Step 2: Odd elements (1,3,5) and even elements (2,4,6) form two separate classes. Step 3: A modulo (2) relation splits a set into odd and even classes.

Correct answer: A. The intersection of two distinct classes is empty

Step 1: Equivalence classes form a partition of the set. Step 2: In a partition, distinct classes do not overlap. Step 3: If two classes overlap, they are not distinct; they are equal.

Correct answer: A. (1)

Step 1: In (A\times A), every element is related to every element. Step 2: Hence all elements fall into one large equivalence class. Step 3: A universal relation on a non-empty set has only one class.

Correct answer: A. ({1},{2},{3},{4})

Step 1: In the identity relation, every element is related only to itself. Step 2: Hence each element forms a separate singleton class. Step 3: The number of classes in an identity relation equals the number of elements.

Correct answer: A. Equivalence relation

Step 1: Every integer is either positive or not positive, so it is related to itself within its group. Step 2: Being in the same group is symmetric. Step 3: It creates two classes: positive integers and non-positive integers.

Correct answer: A. All non-positive integers

Step 1: (0) is not positive. Step 2: Therefore (0) is related to all non-positive integers. Step 3: Read the grouping condition carefully to identify the class.

Correct answer: A. Relation of having the same colour

Step 1: Every object has the same colour as itself. Step 2: If one object has the same colour as another, the reverse is also true. Step 3: Having the same colour is transitive, so it is an equivalence relation.

Correct answer: A. Equivalence relation

Step 1: (\lfloor a\rfloor=\lfloor a\rfloor), so reflexivity holds. Step 2: Equality of the greatest integer value is symmetric. Step 3: If two values are equal to the same middle value, they are equal to each other, so transitivity holds.

Correct answer: A. ([2,3))

Step 1: (\lfloor 2.7\rfloor=2). Step 2: Real numbers with greatest integer value (2) lie in ([2,3)). Step 3: For floor-based classes, the left endpoint is included and the right endpoint is excluded.