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Geometry Difficulty 7.3 National olympiad, round 2 Prove it

The acute triangle ABCABC (AC>BC)(AC> BC) is inscribed in a circle with the center at the point OO, and CDCD is the diameter of this circle. The point KK is on the continuation of the ray DADA beyond the point AA. And the point LL is on the segment BDBD (DL>LB)(DL> LB) so that OKD=BAC\angle OKD = \angle BAC, OLD=ABC\angle OLD = \angle ABC. Prove that the line KLKL passes through the midpoint of the segment ABAB.

Solution

1. Given Information and Initial Setup:
- Triangle ABCABC is acute with AC>BCAC > BC.
- ABCABC is inscribed in a circle with center OO.
- CDCD is the diameter of the circle.
- Point KK is on the extension of ray DADA beyond AA.
- Point LL is on segment BDBD such that DL>LBDL > LB.
- OKD=BAC\angle OKD = \angle BAC and OLD=ABC\angle OLD = \angle ABC.

2. Angle Chasing and Congruence:
- Denote ACB=α\angle ACB = \alpha and ABC=β\angle ABC = \beta.
- Let NN be the intersection of CDCD with the circle centered at OO and radius OKOK.
- By angle chasing, we have NOD=BOL=COK=βα\angle NOD = \angle BOL = \angle COK = \beta - \alpha.

3. Similarity of Triangles:
- Since ODB=OBD=α\angle ODB = \angle OBD = \alpha and ONK=OKN=α\angle ONK = \angle OKN = \alpha, we have OKN{D}ODB{L}\triangle OKN \cup \{D\} \sim \triangle ODB \cup \{L\}.
- This similarity gives us the ratio DLLB=DKDN=DKKC\frac{DL}{LB} = \frac{DK}{DN} = \frac{DK}{KC}.

4. Application of Menelaus' Theorem:
- Let MM be the intersection of line KLKL with segment ABAB.
- Applying Menelaus' theorem in BCD\triangle BCD with transversal KMLKML, we get:
DLLBBMMCKCDK=1 \frac{DL}{LB} \cdot \frac{BM}{MC} \cdot \frac{KC}{DK} = 1
- Using the ratio DLLB=DKKC\frac{DL}{LB} = \frac{DK}{KC}, we substitute into the Menelaus' equation:
DKKCBMMCKCDK=1    BMMC=1 \frac{DK}{KC} \cdot \frac{BM}{MC} \cdot \frac{KC}{DK} = 1 \implies \frac{BM}{MC} = 1
- This implies BM=MCBM = MC, meaning MM is the midpoint of ABAB.

Conclusion:
The line KL passes through the midpoint of segment AB. \boxed{\text{The line } KL \text{ passes through the midpoint of segment } AB.}

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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.