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Matrices & Determinants
Matrices and Determinants
Easy

Question

The number of elements in the set \left\{ {A = \left( {\matrix{ a & b \cr 0 & d \cr } } \right):a,b,d \in \{ - 1,0,1\} \,and\,{{(I - A)}^3} = I - {A^3}} \right\}, where I is 2 ×\times 2 identity matrix, is :

Answer: 3

Solution

This problem tests your understanding of matrix algebra, specifically the binomial expansion for matrices and the properties of idempotent matrices. We are given a condition involving a matrix AA and need to find the number of such matrices whose elements are restricted to {1,0,1}\{-1, 0, 1\}.


1. Key Concept: Binomial Expansion for Matrices

For any two matrices XX and YY that commute (i.e., XY=YXXY = YX), the binomial expansion formula holds: (X±Y)n=k=0n(nk)Xnk(±Y)k(X \pm Y)^n = \sum_{k=0}^n \binom{n}{k} X^{n-k} (\pm Y)^k In our case, we have (IA)3(I - A)^3. Since the identity matrix II commutes with any

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