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Geometry Difficulty 7.5 National olympiad, round 2 Prove it

Let II be the incenter of triangle ABCABC. Let BIBI and ACAC intersect at EE, and CICI and ABAB intersect at FF. Suppose that RR is another intersection of (ABC)\odot (ABC) and (AEF)\odot (AEF). Let MM be the midpoint of BCBC, and P,QP, Q are the intersections of AI,MIAI, MI and EFEF, respectively. Show that A,P,Q,RA, P, Q, R are concyclic.

(ltf0501).

Solution

1. Lemma Proof:
We start by proving the lemma that states: In triangle ABCABC with incenter II, let BIBI intersect ACAC at EE and CICI intersect ABAB at FF. Let MM be the midpoint of BCBC. Then the AA-symmedian, EFEF, and IMIM are concurrent.

To prove this, we use barycentric coordinates. Let the coordinates of AA, BB, and CC be (1,0,0)(1,0,0), (0,1,0)(0,1,0), and (0,0,1)(0,0,1) respectively. The incenter II has coordinates (a:b:c)(a:b:c). The coordinates of EE and FF can be determined as follows:
- EE lies on BIBI and ACAC, so its coordinates are (0:b:c)(0:b:c).
- FF lies on CICI and ABAB, so its coordinates are (a:0:c)(a:0:c).

The midpoint MM of BCBC has coordinates (0:1:1)(0:1:1).

Now, let XX be the point of intersection of the AA-symmedian and IMIM. Assume XX has coordinates (x:y:z)(x:y:z). Since XX lies on the AA-symmedian, we have y:z=b2:c2y:z = b^2:c^2, implying z=c2yb2z = \frac{c^2y}{b^2}.

Since XX also lies on IMIM, we have:
0=xyzabc011=x(bc)ya+ac2yb2 0 = \begin{vmatrix} x & y & z \\ a & b & c \\ 0 & 1 & 1 \end{vmatrix} = x(b-c) - ya + \frac{ac^2y}{b^2}
Simplifying, we get:
x(bc)=y(aac2b2)    x:y=a(b2c2)b2(bc)=a(b+c)b2 x(b-c) = y \left(a - \frac{ac^2}{b^2}\right) \implies x:y = \frac{a(b^2-c^2)}{b^2(b-c)} = \frac{a(b+c)}{b^2}
Thus, the coordinates of XX are (ab+ac:b2:c2)(ab+ac:b^2:c^2).

To prove XX, EE, and FF are collinear, we need the determinant to be zero:
a0cab0ab+acb2c2=abc2+c(ab2ab2abc)=0 \begin{vmatrix} a & 0 & c \\ a & b & 0 \\ ab+ac & b^2 & c^2 \end{vmatrix} = abc^2 + c(ab^2 - ab^2 - abc) = 0
Hence, XX, EE, and FF are collinear.

2. Main Problem:
Given the lemma, we know that AQAQ is the AA-symmedian of ΔABC\Delta ABC. Let APAP and AQAQ intersect (ABC)\odot(ABC) at TT and VV respectively, and let EFEF intersect (ABC)\odot(ABC) at XX and YY. Let VTVT intersect BCBC at KK.

Since 1=(A,V;B,C)=T(ATBC,K;B,C)-1 = (A, V; B, C) \overset{T}{=} (\overline{AT} \cap \overline{BC}, K; B, C), it follows that KXYK \in \overline{XY}. Thus, (B,C)(B, C), (X,Y)(X, Y), and (T,V)(T, V) are pairs of an involution Φ\Phi. Projecting Φ\Phi from AA onto EF\overline{EF}, we get that (X,Y)(X, Y), (E,F)(E, F), and (P,Q)(P, Q) are pairs of an involution. Hence, (ABC)\odot(ABC), (AEF)\odot(AEF), and (APQ)\odot(APQ) are coaxial.

Therefore, AA, PP, QQ, and RR are concyclic. \blacksquare

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.