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MISCELLANEOUS EXERCISE 4 (II) · Q99

Q.Prove by method of induction (3−41−1)n=(2n+1−4nn−2n+1)\begin{pmatrix} 3 & -4 \\ 1 & -1 \end{pmatrix}^n = \begin{pmatrix} 2n+1 & -4n \\ n & -2n+1 \end{pmatrix}, ∀n∈N\forall n \in N.

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Let A=(3−4\1−1)A=\begin{pmatrix}3&-4\1&-1\end{pmatrix}, P(n):An=(2n+1−4n\n−2n+1)P(n):A^n=\begin{pmatrix}2n+1&-4n\n&-2n+1\end{pmatrix}. Base: n=1n=1: R.H.S.=(3−4\1−1)=A=\begin{pmatrix}3&-4\1&-1\end{pmatrix}=A; holds. Hypothesis: assume Ak=(2k+1−4k\k−2k+1)A^k=\begin{pmatrix}2k+1&-4k\k&-2k+1\end{pmatrix}. Step: Ak+1=AkA=(2k+1−4k\k−2k+1)(3−4\1−1)A^{k+1}=A^k A=\begin{pmatrix}2k+1&-4k\k&-2k+1\end{pmatrix}\begin{pmatrix}3&-4\1&-1\end{pmatrix}. Entry(1,1)=3(2k+1)−4k=6k+3−4k=2k+3=2(k+1)+1=3(2k+1)-4k=6k+3-4k=2k+3=2(k+1)+1. Entry(1,2)=−4(2k+1)+4k=−8k−4+4k=−4k−4=−4(k+1)=-4(2k+1)+4k=-8k-4+4k=-4k-4=-4(k+1). Entry(2,1)=3k+(−2k+1)=k+1=3k+(-2k+1)=k+1. Entry(2,2)=−4k−(−2k+1)=−4k+2k−1=−2k−1=−2(k+1)+1=-4k-(-2k+1)=-4k+2k-1=-2k-1=-2(k+1)+1. So $A^{k+1}=\begi …

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