Y by Adventure10, Mango247
Let
be the orthocentre and
be the circumcentre of an acute triangle
. Let
and
be the altitudes of the triangle with
on
and
on
. Let
. Let
be the second point of intersection of the circumcircles of triangles
and
, and let
be the midpoint of side
. Prove that points
are collinear if and only if
is the circumcentre of triangle
.

















