91. JBMO 2010, Problem 3.
by Virgil Nicula, Aug 27, 2010, 5:29 PM
http://www.artofproblemsolving.com/Forum/viewtopic.php?p=1915250&sid=5954534452a76146ace61a65ec21fa85#p1915250
Remark. An easy and nice proposed problem ! Observe that always
. Indeed,

. Otherwise,
what is truly always !
Proof Is well-known that "in any nonisosceles triangle the intersection point between the bisector of an angle of triangle and the perpendicular bisector of the opposite side belongs to the circumcircle of the given triangle". Apply this property to the triangle
and the vertex
. Thus obtain that the quadrilateral
is cyclically
the quadrilateral
is cyclically
. Since
obtain
. In conclusion
.
Remark. In the particular case when
obtain the proposed problem. Another interesting particular case obtain such : denote
,
,
and
. Then for
obtain
. This results from the well-known relation
.
Quote:
Let
and
be angle bisectors in the non-isosceles triangle
, where
and
. The perpendicular bisector of
intersects the line
at point
. Point
lies on the line
such that
is parallel to
. Prove that
.



















Quote:
An easy extension. Let
be the angle bisector in the non-isosceles triangle
, where
and let
be a point for which denote
. The perpendicular bisector of
intersects the line
at the point
. The point
lies on the line
such that
. Prove that
.


























Remark. In the particular case when








This post has been edited 3 times. Last edited by Virgil Nicula, Nov 23, 2015, 2:02 PM