Question:

If A=[1001],P=[1101]A=\begin{bmatrix}1 & 0 \\ 0 & -1\end{bmatrix}, P=\begin{bmatrix}1 & 1 \\ 0 & 1\end{bmatrix} and X=APATX=A P A^{T}, then ATX50A=A^{T} X^{50} A=

Updated On: May 21, 2024
  • [0110] \begin{bmatrix} 0 & 1 \\ 1&0\end{bmatrix}
  • [2101] \begin{bmatrix} 2 &1 \\ 0&-1\end{bmatrix}
  • [251125] \begin{bmatrix}25 & 1\\ 1&- 25 \end{bmatrix}
  • [15001] \begin{bmatrix} 1& 50\\ 0& 1\end{bmatrix}
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The Correct Option is D

Solution and Explanation

Given matrix A=[1001]A=\begin{bmatrix}1 & 0 \\ 0 & -1\end{bmatrix}
is orthogonal matrix, because AAT=IA A^{T}=I.
So, ATX50A=ATX49(APAT)AA^{T} X^{50} A=A^{T} X^{49}\left(A P A^{T}\right) A
=ATX49AP(ATA)=A^{T} X^{49} A P\left(A^{T} A\right)
=ATX49AP=A^{T} X^{49} A P
=ATX48(APAT)AP=ATX48AP2...=A^{T} X^{48}\left(A P A^{T}\right) A P=A^{T} X^{48} A P^{2}...
...=ATAP50=IP50=P50...=A^{T} A P^{50}=I P^{50}=P^{50}
P=[1101]\because P=\begin{bmatrix} 1 & 1 \\ 0 & 1 \end{bmatrix}
P2=[1201]\Rightarrow P^{2}=\begin{bmatrix} 1 & 2 \\ 0 & 1 \end{bmatrix}
P3=[1301]...\Rightarrow P^{3}=\begin{bmatrix} 1 & 3 \\ 0 & 1 \end{bmatrix}...
P50=[15001]\Rightarrow P^{50}=\begin{bmatrix} 1 & 50 \\ 0 & 1 \end{bmatrix}
So, ATX50A=P50=[15001]A^{T} X^{50} A=P^{50}=\begin{bmatrix} 1 & 50 \\ 0 & 1 \end{bmatrix}
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