Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2017/paper-2/17/a/solution

First divide each of the first two equations by two. Then
The equal-component and opposite-component directions diagonalize the upper block, while the third coordinate is independent. Direct multiplication gives the eigenvalues and unit eigenvectors
They form an orthonormal eigenbasis because is a real symmetric matrix.

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