MathDB
Perpendicular diagonals

Source: 2015 BAMO-8 #4, 2015 BAMO-12 #2

February 22, 2016
geometryquadrilateralmidpointslengthsOlympiadB8prove

Problem Statement

In a quadrilateral, the two segments connecting the midpoints of its opposite sides are equal in length. Prove that the diagonals of the quadrilateral are perpendicular.
(In other words, let M,N,P,M,N,P, and QQ be the midpoints of sides AB,BC,CD,AB,BC,CD, and DADA in quadrilateral ABCDABCD. It is known that segments MPMP and NQNQ are equal in length. Prove that ACAC and BDBD are perpendicular.)