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locus of midpoints in 3D

Source: 1961-62 Germany R4 12.5 https://artofproblemsolving.com/community/c3208025_

October 9, 2024
geometryLocus3D geometry

Problem Statement

Given a plane PP and two fixed points AA and BB that do not lie in this plane. Denote two points AA' and BB' on plane PP and M,NM ,N the midpoints of the segments AAAA', BBBB'. a) Determine the locus of the midpoint of the segment MN if the points are AA' and BB' move arbitrarily in plane PP. b) A circle OO is considered in the plane PP. Determine the locus LL of the midpoint of the segment MNMN if the points AA' and BB' lie on the circle OO or inside it . c) AA' is assumed to be fixed on the circle OO or inside it and BB' is assumed to be movable inside it , except for OO. Determine the locus of the point BB' such the above certain locus LL remains the same . Note: For b) and c) the following cases should be considered: 1. AA' and BB' are different, 2. AA' and BB' coincide.