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JEE Advanced 2022 · Matrices

If M=(5232−32−12)M = \begin{pmatrix} \dfrac{5}{2} & \dfrac{3}{2} \\[6pt] -\dfrac{3}{2} & -\dfrac{1}{2} \end{pmatrix}, then which of the following matrices is equal to M2022M^{2022}?

Answer: A. (30343033−3033−3032)\begin{pmatrix} 3034 & 3033 \\ -3033 & -3032 \end{pmatrix}

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Written solution

M=(5232−32−12)M = \begin{pmatrix} \tfrac{5}{2} & \tfrac{3}{2} \\ -\tfrac{3}{2} & -\tfrac{1}{2} \end{pmatrix} To find: M2022\text{To find: } M^{2022} 1. tr⁡(M),det⁡(M)⇒characteristic equation1.\ \operatorname{tr}(M), \det(M) \Rightarrow \text{characteristic equation} 2. show (M−I)2=O2.\ \text{show } (M-I)^2 = O 3. Mn=(I+N)n=I+nN,N=M−I3.\ M^n = (I + N)^n = I + nN, \quad N = M - I tr⁡(M)=52−12=2,det⁡(M)=52(−12)−32(−32)=1\operatorname{tr}(M) = \dfrac{5}{2}-\dfrac{1}{2} = 2, \qquad \det(M) = \dfrac{5}{2}\left(-\dfrac{1}{2}\right) - \dfrac{3}{2}\left(-\dfrac{3}{2}\right) = 1 λ2−2λ+1=0⇒(λ−1)2=0\lambda^2 - 2\lambda + 1 = 0 \Rightarrow (\lambda-1)^2 = 0 (M−I)2=O ⇒ M=I+N,N=M−I,N2=O(M-I)^2 = O \ \Rightarrow \ M = I + N, \quad N = M - I, \quad N^2 = O N=M−I=(3232−32−32)=32(11−1−1)N = M - I = \begin{pmatrix} \tfrac{3}{2} & \tfrac{3}{2} \\ -\tfrac{3}{2} & -\tfrac{3}{2} \end{pmatrix} = \dfrac{3}{2}\begin{pmatrix} 1 & 1 \\ -1 & -1 \end{pmatrix} N2=94(11−1−1)2=ON^2 = \dfrac{9}{4}\begin{pmatrix} 1 & 1 \\ -1 & -1 \end{pmatrix}^2 = O Mn=(I+N)n=I+nN(since N2=O)M^n = (I+N)^n = I + nN \quad (\text{since } N^2 = O) Mn=I+nN=(1+3n23n2−3n21−3n2)M^n = I + nN = \begin{pmatrix} 1 + \tfrac{3n}{2} & \tfrac{3n}{2} \\[2pt] -\tfrac{3n}{2} & 1 - \tfrac{3n}{2} \end{pmatrix} n=2022:3n2=3×20222=3033n = 2022: \quad \dfrac{3n}{2} = \dfrac{3 \times 2022}{2} = 3033 M2022=(30343033−3033−3032) ⇒ option (1)M^{2022} = \begin{pmatrix} 3034 & 3033 \\ -3033 & -3032 \end{pmatrix} \ \Rightarrow \ \text{option } (1)

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