# GATE2017 ME-2: 28

Consider the matrix $A=\begin{bmatrix} 50 &70 \\ 70 & 80 \end{bmatrix}$ whose eigenvectors corresponding to eigenvalues $\lambda _{1}$ and $\lambda _{2}$ are $x_{1}=\begin{bmatrix} 70 \\ \lambda_{1}-50 \end{bmatrix}$ and $x_{2}=\begin{bmatrix} \lambda _{2}-80\\ 70 \end{bmatrix}$, respectively. The value of $x^{T}_{1} x_{2}$ is _________.

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1 vote
Answer should be $0.$

“Eigen-vectors of a real symmetric matrix are orthogonal(perpendicular).”

So, here matrix $A$ is symmetric because $A^T = A.$

So, $x_{1} \perp x_{2}$ (or) $x_{1}^{T} x_{2}=0$ (or) $\vec{x_{1}}.\vec{x_{_{2}}} = 0$ i.e. $(70\hat{i}+(\lambda_{1}-50 )\hat{j}).((\lambda_{2}-80 )\hat{i}+70\hat{j})=0$ i.e $70(\lambda _{2}-80) + 70(\lambda _{1}-50)=0$
410 points 1 2 4

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