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Miscellaneous Exercise 4(B) · Q201

Q.If A=[0110]A=\begin{bmatrix}0 & 1\\1 & 0\end{bmatrix} and B=[0−110]B=\begin{bmatrix}0 & -1\\1 & 0\end{bmatrix} show that (A+B)(A−B)≠A2−B2(A+B)(A-B)\neq A^2-B^2.

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A=[0110]A=\begin{bmatrix}0 & 1\\1 & 0\end{bmatrix}, B=[0−110]B=\begin{bmatrix}0 & -1\\1 & 0\end{bmatrix}.

A+B=[0020]A+B=\begin{bmatrix}0 & 0\\2 & 0\end{bmatrix}, A−B=[0200]A-B=\begin{bmatrix}0 & 2\\0 & 0\end{bmatrix}.

(A+B)(A−B)=[0020][0200]=[0004](A+B)(A-B)=\begin{bmatrix}0 & 0\\2 & 0\end{bmatrix}\begin{bmatrix}0 & 2\\0 & 0\end{bmatrix}=\begin{bmatrix}0 & 0\\0 & 4\end{bmatrix}.

A2=[0110][0110]=[1001]=IA^2=\begin{bmatrix}0 & 1\\1 & 0\end{bmatrix}\begin{bmatrix}0 & 1\\1 & 0\end{bmatrix}=\begin{bmatrix}1 & 0\\0 & 1\end{bmatrix}=I; B2=[0−110][0−110]=[−100−1]=−IB^2=\begin{bmatrix}0 & -1\\1 & 0\end{bmatrix}\begin{bmatrix}0 & -1\\1 & 0\end{bmatrix}=\begin{bmatrix}-1 & 0\\0 & -1\end{bmatrix}=-I.

A2−B2=I−(−I)=2I=[2002]A^2-B^2=I-(-I)=2I=\begin{bmatrix}2 & 0\\0 & 2\end{bmatrix}. …

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