Q84 / 155 · WBJEE 2026 · Mathematics

Let us define the power of a matrix $A$ as the minimum $m \in Z^+$ such that $A^m = I$. For two matrices $A$ and $B$ if $A^5 = I$ and $ABA^{-1} = B^2$, then the power of the matrix $B$ is between

  • A
    20 and 24
  • Correct answer
    B
    28 and 32
  • C
    36 and 40
  • D
    4 and 8

Solution

Given $A B A^{-1} = B^2$

Pre-multiplying by $A$ and post-multiplying by $A^{-1}$, we get:

$A^2 B A^{-2} = A B^2 A^{-1} = (A B A^{-1})^2 = (B^2)^2 = B^4$

Repeating this process $k$ times yields:

$A^k B A^{-k} = B^{2^k}$

Substituting $k = 5$, we obtain:

$A^5 B A^{-5} = B^{2^5} = B^{32}$

Since $A^5 = I$, it follows that $A^{-5} = I$. Substituting this into the equation gives:

$I B I = B^{32} \Rightarrow B = B^{32}$

Since the power of $B$ is defined as the minimum $m \in \mathbb{Z}^+$ such that $B^m = I$, the matrix $B$ must be invertible. Multiplying both sides by $B^{-1}$ gives:

$B^{31} = I$

Since $31$ is a prime number, the minimum positive integer $m$ such that $B^m = I$ is $31$ (for $B \neq I$).

The value $31$ lies between $28$ and $32$.

Answer: 28 and 32

Hence, the correct answer is option: 'B'.

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