 Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2025/paper-1/17/d/solution

tr M = M_{ii} = δ_{ii} - \overset{n}{^}_{i} \overset{n}{^}_{i} = 3 - ∥ \hat{n} ∥^{2} = 2 .

The unit vector condition is essential in the last step.

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