Y by HWenslawski, Adventure10, Mango247
Let
be the midpoint of the side
of a triangle
and
intersect the circumcircle of
for the second time at
. Let
be the point symmetric to the point
with respect to the point
and
be the point of intersection of the lines
and
. Prove that if
are concyclic, then the lines
and
are perpendicular.














