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 P: 87 Given a symmetric matrix $$A=\left(\begin{array}{ccccc} \sum a_{1s} & & & & \\ & \ddots & & a_{ij} \\ & & \ddots & & \\ &a_{ij} & & \ddots & \\ & & & & \sum w_{as} \end{array}\right) \in\mathbb{R}^{n\times n},$$ with strictly positive entries a_{ij}, and with the diagonal entries being sum of off-diagonal entries residing in the corresponding row/column, how to proceed with the proof for A being positive definite, $$x^TAx>0$$ for some non-zero vector x.