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The linear variational method diagonalizes [abbd]\begin{bmatrix}a & b \\ b & d\end{bmatrix} with a=1a = 1, b=1b = 1, d=3d = 3. The lower eigenvalue is mean((ad)/2)2+b2\text{mean} - \sqrt{((a-d)/2)^2 + b^2}. What is it?