Let be the incenter of triangle . Let and intersect at , and and intersect at . Suppose that is another intersection of and . Let be the midpoint of , and are the intersections of and , respectively. Show that are concyclic.
(ltf0501).
Let be the incenter of triangle . Let and intersect at , and and intersect at . Suppose that is another intersection of and . Let be the midpoint of , and are the intersections of and , respectively. Show that are concyclic.
(ltf0501).
1. Lemma Proof:
We start by proving the lemma that states: In triangle with incenter , let intersect at and intersect at . Let be the midpoint of . Then the -symmedian, , and are concurrent.
To prove this, we use barycentric coordinates. Let the coordinates of , , and be , , and respectively. The incenter has coordinates . The coordinates of and can be determined as follows:
- lies on and , so its coordinates are .
- lies on and , so its coordinates are .
The midpoint of has coordinates .
Now, let be the point of intersection of the -symmedian and . Assume has coordinates . Since lies on the -symmedian, we have , implying .
Since also lies on , we have:
Simplifying, we get:
Thus, the coordinates of are .
To prove , , and are collinear, we need the determinant to be zero:
Hence, , , and are collinear.
2. Main Problem:
Given the lemma, we know that is the -symmedian of . Let and intersect at and respectively, and let intersect at and . Let intersect at .
Since , it follows that . Thus, , , and are pairs of an involution . Projecting from onto , we get that , , and are pairs of an involution. Hence, , , and are coaxial.
Therefore, , , , and are concyclic.