Question: Coordinates of a point (9)

Please write for me a code for the following problem.

Let (S): x^2 + y^2 + z^2 -2*x +2*z + 1 = 0 be a sphere and M(1, 2, -1), N(3, 1, -1) be two points. Find the coordinates of the point K lies on the sphere (S) such that the triangle KHN has minimum area. 

Thank you very much.

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