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Geometry Difficulty 6.6 National olympiad Prove it Greece

Let ABΓAB\Gamma be an acute angled triangle with AB<AΓ<BΓAB < A\Gamma < B\Gamma and let cc be its circumcircle with centre OO. At the small arcs AΓA\Gamma and ABAB we consider the points Δ\Delta and EE respectively. Let KK be the intersection point of BΔB\Delta, ΓE\Gamma E and NN be the second common point of the circumcircles of the triangles BKEBKE, let it be c1c_1, and ΓKΔ\Gamma K\Delta, let it be c2c_2. Prove that the points A,K,NA, K, N are collinear if and only if the point KK lies on the A-symmedian of the triangle ABΓAB\Gamma.

Solutions — 2

Solution 1

Let KK be the intersection point of BΔB\Delta, ΓE\Gamma E and NN be the second intersection point of c1c_1, c2c_2. Let ZZ be the point of intersection of the tangents at B,ΓB, \Gamma of the circle cc. We will prove that the points K,N,ZK, N, Z are collinear.
Let TT be the intersection of BZBZ with c1c_1 and PP the intersection of ZΓZ\Gamma with c2c_2, then:
E^1=T^1:(inscribed in c1 at the same arc BK) \hat{E}_1 = \hat{T}_1 : (\text{inscribed in } c_1 \text{ at the same arc } BK)
E^1=B^1:(chord and tangent at c) \hat{E}_1 = \hat{B}_1 : (\text{chord and tangent at } c)
Δ^1=P^1:(inscribed in c2 at the same arc ΓK) \hat{\Delta}_1 = \hat{P}_1 : (\text{inscribed in } c_2 \text{ at the same arc } \Gamma K)
Δ^1=Γ^1:(chord and tangent at c) \hat{\Delta}_1 = \hat{\Gamma}_1 : (\text{chord and tangent at } c)
B^1=Γ^1:(ZB and Z\Gammaare tangents at c) \hat{B}_1 = \hat{\Gamma}_1 : (\text{ZB and Z\Gamma are tangents at } c)
From the above inequalities we have:
B^1=T^1, so KT//BΓ and P^1=Γ^1, so KP//BΓ. \hat{B}_1 = \hat{T}_1, \text{ so } KT // B\Gamma \text{ and } \hat{P}_1 = \hat{\Gamma}_1, \text{ so } KP // B\Gamma.
Therefore BΓPTB\Gamma PT is an isosceles trapezium and thus it is cyclic (let c3c_3 its circle).
This means that the common chord KNKN of the circles c1c_1 and c2c_2 will pass through the radical center ZZ of the circles c1,c2,c3c_1, c_2, c_3.
Suppose first that A,K,NA, K, N are collinear. Then, since K,N,TK, N, T are collinear, we have that A,K,N,TA, K, N, T are collinear, so A,K,NA, K, N are on the symmedian ATAT.
Conversely, if KK is on the symmedian ATAT, then A,K,TA, K, T are collinear and since K,N,TK, N, T are collinear, we conclude that A,K,NA, K, N are collinear.

Solution 2

Let KK be the intersection point of BΔB\Delta, ΓE\Gamma E and NN the second common point of c1,c2c_1, c_2. Let ZZ be the point of intersection of the tangents at B,ΓB, \Gamma of the circle cc. If HH is the intersection of EBEB and ΔΓ\Delta\Gamma, we will prove that K,N,Z,HK, N, Z, H are collinear.
Using Pascal's theorem at the degenerate hexagon BBΓΓΔEBB\Gamma\Gamma\Delta E, we have that H,K,ZH, K, Z are collinear.
The point HH has power with respect to cc, HEHB=HΔHΓHE \cdot HB = H\Delta \cdot H\Gamma.
Therefore K,N,HK, N, H are collinear. Therefore K,N,Z,HK, N, Z, H are collinear.

Now it is clear that A,K,NA, K, N are collinear if and only if KK is on the symmedian AZAZ.

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