2021 IMO Problems/Problem 4
Let
be a circle with centre
, and
a convex quadrilateral such that each of
the segments
and
is tangent to
. Let
be the circumcircle of the triangle
.
The extension of
beyond
meets
at
, and the extension of
beyond
meets
at
.
The extensions of
and
beyond
meet
at
and
, respectively. Prove that
Let be the centre of
For
the result follows simply. By Pitot's Theorem we have
so that,
The configuration becomes symmetric about
and the result follows immediately.
Now assume WLOG . Then
lies between
and
in the minor arc
and
lies between
and
in the minor arc
.
Consider the cyclic quadrilateral
.
We have
and
. So that,
. Since
is the incenter of quadrilateral
,
is the angular bisector of
. This gives us,
. Hence the chords
and
are equal.
So
is the reflection of
about
.
Similarly we get
and so the chords
and
are equal. Hence
is the reflection of
about
.
This gives us
=
immediately and now it suffices to prove,
.