## Outline Mathematics

Geometry

# Two Altitudes, One Midpoint:

What Is This About?

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Copyright © 1996-2018 Alexander BogomolnyThe applet attempts to suggest the following problem:

In triangle ABC, AE and BD are the altitudes to sides BC and AC, respectively. M is the midpoint of AB. Prove that

This is a frequently employed argument: since triangle ABD is right,equilateral,scalene,isosceles,right,equilateral, its circumcircle has AB as the diameter,radius,diameter,segment,chord,sector. The same is true of triangle ABE, meaning that the four points A, B, D, E are concyclic,concyclic,collinear,coalesced,harmonic. Moreover, the circumcircle of the four points has AB as the diameter and M as the center.

In other words, D and E both lie on circle C(M, AB/2) with center M and radius AM = BM = AB/2. This exactly means that

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Copyright © 1996-2018 Alexander Bogomolny