Classic complex number geo
by Ciobi_, Apr 2, 2025, 12:56 PM
Let
be a point in the plane, distinct from the vertices of
. Consider
the reflections of
with respect to lines
and
, in this order.
a) Prove that
are collinear if and only if
lies on the circumcircle of
.
b) If
does not lie on the circumcircle of
and the centroids of triangles
and
coincide, prove that
is equilateral.






a) Prove that



b) If





Olympiad Geometry problem-second time posting
by kjhgyuio, Apr 2, 2025, 1:03 AM
In trapezium ABCD,AD is parallel to BC and points E and F are midpoints of AB and DC respectively. If
Area of AEFD/Area of EBCF =√3 + 1/3-√3 and the area of triangle ABD is √3 .find the area of trapezium ABCD
Area of AEFD/Area of EBCF =√3 + 1/3-√3 and the area of triangle ABD is √3 .find the area of trapezium ABCD
April Fools Geometry
by awesomeming327., Apr 1, 2025, 2:52 PM
Let
be an acute triangle with
, and let
be the projection from
onto
. Let
be a point on the extension of
past
such that
. Let
be on the perpendicular bisector of
such that
and
are on the same side of
and
Let the reflection of
across
and
be
and
, respectively. Let
and
such that
. Let
and
intersect the circumcircles of
and
at
and
, respectively. Let
and
intersect
and
at
and
. Let
intersect
at
. Prove that
.














![\[\frac12\angle ALE=1.4\angle ABE+3.4\angle ACE-558^\circ\]](http://latex.artofproblemsolving.com/c/6/3/c63eeb053f1fa8a29bef93516814ae121382219e.png)
























Assisted perpendicular chasing
by sarjinius, Mar 9, 2025, 3:41 PM
In acute triangle
with circumcenter
and orthocenter
, let
be an arbitrary point on the circumcircle of triangle
such that
does not lie on line
and that line
is not parallel to line
. Let
be the point on the circumcircle of triangle
such that
is perpendicular to
, and let
be the point on line
such that
. Let
and
be the points on the circumcircle of triangle
such that
is a diameter, and
and
are parallel. Let
be the midpoint of
.
(a) Show that
and
are perpendicular.
(b) Show that
and
are perpendicular.
























(a) Show that


(b) Show that


Famous geo configuration appears on the district MO
by AndreiVila, Mar 8, 2025, 1:28 PM
Let
be a convex hexagon with
and
.



- Prove that there is a unique point
which is equidistant from sides
and
.
- If
and
are the centers of mass of
and
, show that
.
Geo Final but hard to solve with Conics...
by Seungjun_Lee, Jan 18, 2025, 7:13 AM
Let
be the circumcircle of triangle
with center
, and the
inmixtilinear circle is tangent to
at
respectively.
is the intersection of
and
and
is the intersection of
and
. Prove that the isogonal conjugate of
lies on the line passing through the midpoint of
and
.















This post has been edited 1 time. Last edited by Seungjun_Lee, Jan 18, 2025, 12:44 PM
calculate the perimeter of triangle MNP
by PennyLane_31, Oct 16, 2024, 8:26 PM
Let
be a convex quadrilateral, and
,
, and
be the midpoints of diagonals
and
, and side
, respectively. Also, suppose that
and that
,
. Calculate the perimeter of triangle
.











IMOC 2017 G2 , (ABC) <= (DEF) . perpendiculars related
by parmenides51, Mar 20, 2020, 9:08 AM
Given two acute triangles
. If
and
, show that the area of
is not less than the area of 





This post has been edited 3 times. Last edited by parmenides51, Jan 2, 2022, 12:07 PM
Reason: huge typo, corrected after #3
Reason: huge typo, corrected after #3
Show that AB/AC=BF/FC
by syk0526, Apr 2, 2012, 3:06 PM
Let
be an acute triangle. Denote by
the foot of the perpendicular line drawn from the point
to the side
, by
the midpoint of
, and by
the orthocenter of
. Let
be the point of intersection of the circumcircle
of the triangle
and the half line
, and
be the point of intersection (other than
) of the line
and the circle
. Prove that
must hold.
(Here we denote
the length of the line segment
.)

















(Here we denote


This post has been edited 5 times. Last edited by syk0526, Apr 4, 2012, 6:48 AM
"Where wisdom and valor fail, all that remains is faith. . . And it can overcome all." -Toa Mata Tahu
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