Let $\mathrm{A}=\left[a_{i j}\right]$ be a matrix of order $3 \times 3$, with $a_{i j}=(\sqrt{2})^{i+j}$. If the sum of all the elements in the third row of $A^2$ is $\alpha+\beta \sqrt{2}, \alpha, \beta \in \mathbf{Z}$, then $\alpha+\beta$ is equal to :
Solution
<p>$$\begin{aligned}
& A=\left[\begin{array}{lll}
(\sqrt{2})^2 & (\sqrt{2})^3 & (\sqrt{2})^4 \\
(\sqrt{2})^3 & (\sqrt{2})^4 & (\sqrt{2})^5 \\
(\sqrt{2})^4 & (\sqrt{2})^5 & (\sqrt{2})^6
\end{array}\right] \\
& A=\left[\begin{array}{ccc}
2 & 2 \sqrt{2} & 4 \\
2 \sqrt{2} & 4 & 4 \sqrt{2} \\
4 & 4 \sqrt{2} & 8
\end{array}\right] \\
& A^2=2^2\left[\begin{array}{ccc}
1 & \sqrt{2} & 2 \\
\sqrt{2} & 2 & 2 \sqrt{2} \\
2 & 2 \sqrt{2} & 4
\end{array}\right]\left[\begin{array}{ccc}
1 & \sqrt{2} & 2 \\
\sqrt{2} & 2 & 2 \sqrt{2} \\
2 & 2 \sqrt{2} & 4
\end{array}\right]
\end{aligned}$$</p>
<p>$$\begin{aligned}
&=4\left[\begin{array}{ccc}
- & - & - \\
- & - & - \\
(2+4+8) & (2 \sqrt{2}+4 \sqrt{2}+8 \sqrt{2}) & (4+8+16)
\end{array}\right]\\
&\text { Sum of elements of } 3^{\text {rd }} \text { row }=4(14+14 \sqrt{2}+28)\\
&\begin{aligned}
& =4(42+14 \sqrt{2}) \\
& =168+56 \sqrt{2} \\
& \alpha+\beta \sqrt{2} \\
\therefore \quad & \alpha+\beta=168+56=224
\end{aligned}
\end{aligned}$$</p>
About this question
Subject: Mathematics · Chapter: Matrices and Determinants · Topic: Types of Matrices and Operations
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