Solution

ID: past-exam-of-the-mathematical-methods-ii-course-of-the-university-of-oxford/2016/8/c/solution

By Stokes theorem, it suffices to calculate the boundary line integral. On the plane, and , so
This exact differential has zero integral around a closed curve. Therefore
The argument applies to any admissible spanning surface, including a curved one; the surface itself need not lie in the plane.

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