Let $A$ be a matrix of order $3 \times 3$ and $|A|=5$. If $|2 \operatorname{adj}(3 A \operatorname{adj}(2 A))|=2^\alpha \cdot 3^\beta \cdot 5^\gamma, \alpha, \beta, \gamma \in N$, then $\alpha+\beta+\gamma$ is equal to
JEE · Math · previous-year question
- A.26
- B.27correct
- C.25
- D.28
Answer
B. 27
Explanation
To find the expression $|2 \operatorname{adj}(3 A \operatorname{adj}(2 A))|$, we break it down as follows: Recognize that: $ |2 \operatorname{adj}(3 A \operatorname{adj}(2 A))| = 2^3 |3A (\operatorname{adj}(2A))|^2 $ Apply properties of determinants: $ = 2^3 (3^3)^2 |A|^2 \left|\operatorname{adj}(2A)\right|^2 $ Further simplify using $|\operatorname{adj}(B)| = |B|^{n-1}$ for a $3 \times 3$ matrix: $ = 2^3 \cdot 3^6 \cdot 5^2 \cdot (|2A|^2)^2 $ Simplify $|2A|$: $ = 2^3 \cdot 3^6 \cdot 5^2 \cdot (2^3)^4 \cdot |A|^4 $ Continue to simplify: $ = 2^3 \cdot 3^6 \cdot 5^2 \cdot (2^3)^4 \cdot 5^4 $ Expand and combine powers: $ = 2^{15} \cdot 3^6 \cdot 5^6 $ Therefore, $\alpha = 15$, $\beta = 6$, and $\gamma = 6$. So, $\alpha + \beta + \gamma = 15 + 6 + 6 = 27$.
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