COMEDK · Maths · 17. Mathematical Reasoning

Which of the following is true?

A The set of all fourth roots of unity is a multiplicative group
B The set of all cube roots of unity is an additive group
C \((a b)^{-1}=a^{-1} b^{-1}\) for all \(a, b\) in any group \(G\)
D If \((a b)^{2}=a^{2} b^{2}\) for all \(a, b\) in any group \(G\), then the group \(G\) is non-abelian

Verified Solution

Answer & Solution

Correct Answer

(A) The set of all fourth roots of unity is a multiplicative group

Step-by-step Solution

Detailed explanation

(a) Set of all fourth roots of unity is \[ \begin{aligned} &a=\{1,-1, i,-i\} \\ &\text { Now, } 1 \times(-1)=-1 \in G \\ &\quad 1 \times i=i \in G \\ &1 \times(-i)=-i \in G \\ &(-1) \times i=-i \in G \\ &(-1) \times(-i)=i \in G \\ &i \times(-i)=-i^{2}=1 \in G \end{aligned} \]…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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