Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2012/paper-2/11/c/solution

Choose two perpendicular unit directions in the eigenspace:
Their pairwise dot products vanish and their Euclidean norms are one. They form an orthonormal basis of eigenvectors, with eigenvalues , as guaranteed for a real symmetric matrix by the spectral theorem for real symmetric matrices.

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