Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/ib/paper-3/12e/i/solution

Take the parallel at height one,
It has constant speed . Its distance from the axis is the constant , and its tangent is parallel to the parallel, so and it satisfies the Clairaut first integral for a surface of revolution.
However,
whereas a normal vector to at is
These vectors are not parallel. By the ambient acceleration criterion for a surface geodesic, is not a geodesic. Thus constant speed together with Clairaut's relation is not sufficient for the geodesic equations.

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