Maths Olympiad Prep

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Geometry Difficulty 6.7 National olympiad Prove it

Given is circle ω\omega with diameter AKA K. Point MM lies inside the circle, not on line AKA K. The line AMA M intersects ω\omega again at QQ. The tangent to ω\omega at QQ intersects the line through MM perpendicular to AKA K at PP. Point LL lies on ω\omega such that PLP L is a tangent, with LQL \neq Q. Prove that K,LK, L, and MM lie on a line.
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Solution

Let OO be the center of ω\omega and let VV be the intersection of MPM P with AKA K. We first prove that PVL=POL\angle P V L=\angle P O L. If VV and OO coincide, there is nothing to prove. If VV and OO do not coincide, then OVP=90=OLP\angle O V P=90^{\circ}=\angle O L P, so OVPLO V P L or VOPLV O P L is a cyclic quadrilateral. (In fact, QQ also lies on the corresponding circumscribed circle.) From this, it follows that PVL=POL\angle P V L=\angle P O L. We now have in all cases MVL=PVL=POL\angle M V L=\angle P V L=\angle P O L. Since PLP L and PQP Q are tangents to ω\omega, OQPOLP\triangle O Q P \cong \triangle O L P, so POL=12QOL\angle P O L=\frac{1}{2} \angle Q O L. By the central angle theorem applied to ω\omega, this angle is also equal to QAL\angle Q A L. Altogether, we find

MVL=POL=QAL=MAL \angle M V L=\angle P O L=\angle Q A L=\angle M A L

which implies that MVALM V A L is a cyclic quadrilateral. Therefore, ALM=180AVM=90\angle A L M=180^{\circ}-\angle A V M=90^{\circ}. Furthermore, by Thales' theorem, ALK=90\angle A L K=90^{\circ}, so ALM=ALK\angle A L M=\angle A L K, which means that LL, MM, and KK lie on a straight line.

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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.