Two identical thin plane-convex lenses, the focal lengths of which are equal to F, are placed in a frame so that their convex surfaces touch. Determine the focal length of such a system in water, the refractive index of which is equal to n. Assume that water will not get inside the frame. Consider only those rays that intersect the optical axis at angles small enough so that the tangents of these angles can be approximately replaced by sines.

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