GATE CS 2024 Set 1 — Question 52

MSQ+2 / -0MediumGroups, Rings & FieldsSets & CombinatoricsEngineering MathematicsNumerical Relations & ReasoningAnalytical AptitudeGeneral Aptitude

Engineering Mathematics → Analytical Aptitude → Groups, Rings & Fields

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Question

Consider the operators \diamond and \Box defined by ab=a+2ba \diamond b = a + 2b, ab=aba \Box b = ab, for positive integers. Which of the following statements is/are TRUE?
A.
Operator \diamond obeys the associative law
B.
Operator \Box obeys the associative law
C.
Operator \diamond over the operator \Box obeys the distributive law
D.
Operator \Box over the operator \diamond obeys the distributive law

Correct answer

(B) Operator obeys the associative law; (D) Operator over the operator obeys the distributive law

Solution

Let us analyze each statement:
Statement (A): Associativity of \diamond
Check if (ab)c=a(bc)(a \diamond b) \diamond c = a \diamond (b \diamond c).
LHS: (ab)c=(a+2b)c=(a+2b)+2c=a+2b+2c(a \diamond b) \diamond c = (a + 2b) \diamond c = (a + 2b) + 2c = a + 2b + 2c
RHS: a(bc)=a(b+2c)=a+2(b+2c)=a+2b+4ca \diamond (b \diamond c) = a \diamond (b + 2c) = a + 2(b + 2c) = a + 2b + 4c
Since a+2b+2ca+2b+4ca + 2b + 2c \neq a + 2b + 4c generally, \diamond is NOT associative.
Statement (B): Associativity of \Box
ab=aba \Box b = ab (standard multiplication).
Multiplication of integers is associative: (ab)c=a(bc)(ab)c = a(bc).
Thus, \Box is associative.
Statement (C): Distributivity of \diamond over \Box
Check if a(bc)=(ab)(ac)a \diamond (b \Box c) = (a \diamond b) \Box (a \diamond c).
LHS: a(bc)=a+2bca \diamond (bc) = a + 2bc
RHS: (a+2b)(a+2c)=(a+2b)(a+2c)=a2+2ac+2ab+4bc(a + 2b) \Box (a + 2c) = (a + 2b)(a + 2c) = a^2 + 2ac + 2ab + 4bc
LHS \neq RHS generally. Thus, \diamond does NOT distribute over \Box.
Statement (D): Distributivity of \Box over \diamond
Check if a(bc)=(ab)(ac)a \Box (b \diamond c) = (a \Box b) \diamond (a \Box c).
LHS: a(b+2c)=a(b+2c)=ab+2aca \Box (b + 2c) = a(b + 2c) = ab + 2ac
RHS: (ab)(ac)=ab+2(ac)=ab+2ac(ab) \diamond (ac) = ab + 2(ac) = ab + 2ac
LHS = RHS. Thus, \Box distributes over \diamond.
Therefore, statements (B) and (D) are TRUE.

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