Solution

ID: past-exam-of-the-mathematics-course-of-the-year-2-of-the-natural-sciences-course-of-the-university-of-cambridge/2013/paper-1/9/iii/solution

Here has no explicit dependence, so use the Beltrami identity. With , one has and therefore
This first integral reduces the second-order Euler-Lagrange equation to a first-order relation.

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