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Geometry Difficulty 8.7 Shortlist Prove it IMO

Let OO be the circumcentre, and Ω\Omega be the circumcircle of an acute-angled triangle ABCA B C. Let PP be an arbitrary point on Ω\Omega, distinct from A,B,CA, B, C, and their antipodes in Ω\Omega. Denote the circumcentres of the triangles AOP,BOPA O P, B O P, and COPC O P by OA,OBO_{A}, O_{B}, and OCO_{C}, respectively. The lines A,B\ell_{A}, \ell_{B}, and C\ell_{C} perpendicular to BC,CAB C, C A, and ABA B pass through OA,OBO_{A}, O_{B}, and OCO_{C}, respectively. Prove that the circumcircle of the triangle formed by A,B\ell_{A}, \ell_{B}, and C\ell_{C} is tangent to the line OPO P.

Solution

Let the lines B\ell_{B} and C\ell_{C} meet at LAL_{A}; define the points LBL_{B} and LCL_{C} similarly. Note that the sidelines of the triangle LALBLCL_{A} L_{B} L_{C} are perpendicular to the corresponding sidelines of ABCA B C. Points OA,OB,OCO_{A}, O_{B}, O_{C} are located on the corresponding sidelines of LALBLCL_{A} L_{B} L_{C}; moreover, OA,OBO_{A}, O_{B}, OCO_{C} all lie on the perpendicular bisector of OPO P.

Figure 1

Claim 1. The points LB,P,OAL_{B}, P, O_{A}, and OCO_{C} are concyclic.

Proof. Since OO is symmetric to PP in OAOCO_{A} O_{C}, we have
(O A P, O C P )= (O C O, O A O )= (C P, A P)= (C B, A B)= (O A L B , O C L B ) .\text{(O A P, O C P )= (O C O, O A O )= (C P, A P)= (C B, A B)= (O A L B , O C L B ) .}
Denote the circle through LB,P,OAL_{B}, P, O_{A} and OCO_{C} by ωB\omega_{B}. Define the circles ωA\omega_{A} and ωC\omega_{C} similarly.

Claim 2. The circumcircle of the triangle LALBLCL_{A} L_{B} L_{C} passes through PP.

Proof. From cyclic quadruples of points in the circles ωB\omega_{B} and ωC\omega_{C}, we have
(L C L A , L C P ) = (L C O B , L C P )= (O A O B , O A P ) = (O A O C , O A P )= (L B O C , L B P )= (L B L A , L B P ) .\text{(L C L A , L C P ) = (L C O B , L C P )= (O A O B , O A P ) = (O A O C , O A P )= (L B O C , L B P )= (L B L A , L B P ) .}

Claim 3. The points P,LCP, L_{C}, and CC are collinear.

Proof. We have (P L C , L C L A )= (P L C , L C O B )= (P O A , O A O B )\text{(P L C , L C L A )= (P L C , L C O B )= (P O A , O A O B )}. Further, since OAO_{A} is the centre of the circle AOPA O P, (P O A , O A O B )= (P A, A O)\text{(P O A , O A O B )= (P A, A O)}. As OO is the circumcentre of the triangle P C A, (P A, A O)= / 2- (C A, C P)= (C P, L C L A )\text{P C A, (P A, A O)= / 2- (C A, C P)= (C P, L C L A )}. We obtain (P L C , L C L A )= (C P, L C L A )\text{(P L C , L C L A )= (C P, L C L A )}, which shows that PCLCP \in C L_{C}.

Similarly, the points P,LA,AP, L_{A}, A are collinear, and the points P,LB,BP, L_{B}, B are also collinear. Finally, the computation above also shows that
(O P, P L A )= (P A, A O)= (P L C , L C L A ),\text{(O P, P L A )= (P A, A O)= (P L C , L C L A ),}
which means that OPO P is tangent to the circle PLALBLCP L_{A} L_{B} L_{C}.

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