Let \(y = f(x)\) be any curve on the X-Y plane and \(P\) be a point on the curve. Let \(C\) be a fixed point not on the curve. The length \(PC\) is either a maximum or a minimum. Then:
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When dealing with optimization problems involving distances and curves, the distance is maximized or minimized when the vector from the point of interest is either parallel or perpendicular to the tangent.