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Geometry Difficulty 7.0 National olympiad, round 2 Find the answer

Let ABC\triangle ABC be an acute triangle with circumcenter OO, and let QAQ\neq A denote the point on (ABC)\odot (ABC) for which AQBCAQ\perp BC. The circumcircle of BOC\triangle BOC intersects lines ACAC and ABAB for the second time at DD and EE respectively. Suppose that AQAQ, BCBC, and DEDE are concurrent. If OD=3OD=3 and OE=7OE=7, compute AQAQ.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Identify the key points and properties:
- ABC\triangle ABC is an acute triangle with circumcenter OO.
- QAQ \neq A is a point on (ABC)\odot(ABC) such that AQBCAQ \perp BC.
- The circumcircle of BOC\triangle BOC intersects ACAC and ABAB at DD and EE respectively.
- AQAQ, BCBC, and DEDE are concurrent.
- Given OD=3OD = 3 and OE=7OE = 7.

2. **Establish the cyclic nature of quadrilateral ADQEADQE:**
- Since QQ lies on (ABC)\odot(ABC) and is collinear with BECDBE \cap CD and BCDEBC \cap DE, QQ is the Miquel point of the complete quadrilateral {BE,CD,BC,DE}\{BE, CD, BC, DE\}.
- Therefore, quadrilateral ADQEADQE is cyclic.

3. **Determine the relationship between O2O_2 and QQ:**
- Let O2O_2 be the circumcenter of (BOC)\odot(BOC).
- Since O2QAQO_2Q \perp AQ, the antipode of AA with respect to (ABC)\odot(ABC), denoted as KK, lies on line QO2QO_2.

4. Analyze the orthocenter and circumradius properties:
- Given that BCBC and DEDE are antiparallel with respect to A\angle A, AODEAO \perp DE.
- Angle chasing shows that OO is the orthocenter of ADE\triangle ADE.
- Thus, (ADE)\odot(ADE) and (BOC)\odot(BOC) have the same circumradius.

5. Set up the circumradius relationships:
- Let R1R_1 be the circumradius of (ABC)\odot(ABC) and R2R_2 be the circumradius of (ADE)\odot(ADE).
- We have DO=2R2cosB=3DO = 2R_2 \cos B = 3 and EO=2R2cosC=7EO = 2R_2 \cos C = 7.

6. **Calculate AQAQ:**
- AQ=2OO2=2R2AQ = 2OO_2 = 2R_2.
- Also, AQ=AKcos(BC)=2R1cos(BC)AQ = AK \cos(B - C) = 2R_1 \cos(B - C).
- Using the relationship R2=BC2sin2A=R12cosAR_2 = \frac{BC}{2 \sin 2A} = \frac{R_1}{2 \cos A}, we get cosAcos(BC)=12\cos A \cos(B - C) = \frac{1}{2}.

7. Perform trigonometric manipulations:
- cosAcos(BC)=12\cos A \cos(B - C) = \frac{1}{2}.
- This implies cos(B+C)cos(BC)=12\cos(B + C) \cos(B - C) = -\frac{1}{2}.
- Therefore, cos2B+cos2C=1\cos 2B + \cos 2C = -1.

8. **Solve for R2R_2:**
- Using the given values, 2(32R2)21+2(72R2)21=12 \left(\frac{3}{2R_2}\right)^2 - 1 + 2 \left(\frac{7}{2R_2}\right)^2 - 1 = -1.
- Simplifying, we get:
2(94R22)1+2(494R22)1=1 2 \left(\frac{9}{4R_2^2}\right) - 1 + 2 \left(\frac{49}{4R_2^2}\right) - 1 = -1
184R22+984R222=1 \frac{18}{4R_2^2} + \frac{98}{4R_2^2} - 2 = -1
1164R222=1 \frac{116}{4R_2^2} - 2 = -1
29R22=1 \frac{29}{R_2^2} = 1
R22=29 R_2^2 = 29
R2=29 R_2 = \sqrt{29}

9. **Compute AQAQ:**
- AQ=2R2=229AQ = 2R_2 = 2\sqrt{29}.

The final answer is 229\boxed{2\sqrt{29}}.

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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.