Maths Olympiad Prep

Track / Stage 7 / 182 of 300 #1582 of 1964

Problem 1582

National Olympiad second round; IMO P1/P4
Geometry Difficulty 7.3 Find the answer

Let Γ\Gamma be a circle centered at OO with chord ABAB. The tangents to Γ\Gamma at AA and BB meet at CC. A secant from CC intersects chord ABAB at DD and Γ\Gamma at EE such that DD lies on segment CECE. Given that ∠BOD+∠EAD=180∘\angle BOD + \angle EAD = 180^\circ, AE=1AE = 1, and BE=2BE = 2, find CECE.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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Official solution

1. Identify the given conditions and setup the problem:
- Circle Γ\Gamma is centered at OO.
- Chord ABAB with tangents at AA and BB meeting at CC.
- Secant from CC intersects ABAB at DD and Γ\Gamma at EE such that DD lies on segment CECE.
- Given: ∠BOD+∠EAD=180∘\angle BOD + \angle EAD = 180^\circ, AE=1AE = 1, and BE=2BE = 2.

2. Analyze the given angle condition:
- ∠BOD+∠EAD=180∘\angle BOD + \angle EAD = 180^\circ implies ∠BOD+∠EAB=180∘\angle BOD + \angle EAB = 180^\circ.
- This means ∠BOD=180∘−∠EAB\angle BOD = 180^\circ - \angle EAB.
- Since ∠EAB\angle EAB is an inscribed angle, EAB = EAB 2\text{EAB = EAB 2}.
- Therefore, BOD = 180 - EAB 2 = BOE 2\text{BOD = 180 - EAB 2 = BOE 2}.
- This implies that line ODOD bisects ∠BOE\angle BOE.

3. Use the perpendicular bisector property:
- Since OO is the center of Γ\Gamma, OD‾\overline{OD} is the perpendicular bisector of BEBE.
- By the perpendicularity lemma, BD=EDBD = ED.

4. Establish congruency and harmonic division:
- By ASA congruency, △BFD≅△EAD\triangle BFD \cong \triangle EAD.
- Thus, BF=EA=1BF = EA = 1.
- The tangents at AA and BB intersect at CC on the extension of EF‾\overline{EF}.
- By harmonic division lemma, cyclic quadrilateral AEBFAEBF is harmonic, i.e., (AB;EF)=−1(AB;EF) = -1.
- Therefore, EAEB÷FAFB=−1\frac{EA}{EB} \div \frac{FA}{FB} = -1.
- Given EA=1EA = 1 and EB=2EB = 2, we have 12÷FA1=−1\frac{1}{2} \div \frac{FA}{1} = -1, leading to FA=12FA = \frac{1}{2}.

5. Apply Ptolemy's theorem:
- Since AEBFAEBF is an isosceles trapezoid, by Ptolemy's theorem:
AF⋅BE+AE⋅BF=EF⋅AB AF \cdot BE + AE \cdot BF = EF \cdot AB
- Substituting the known values:
(12)⋅2+1⋅1=EF2 \left(\frac{1}{2}\right) \cdot 2 + 1 \cdot 1 = EF^2
- Simplifying, we get:
1+1=EF2  ⟹  EF2=2  ⟹  EF=2 1 + 1 = EF^2 \implies EF^2 = 2 \implies EF = \sqrt{2}

6. Use the Law of Cosines:
- Let CF=xCF = x. By the Law of Cosines on △ACF\triangle ACF:
AC2=x2+(12)2−2(12)xcos⁡(∠AFC) AC^2 = x^2 + \left(\frac{1}{2}\right)^2 - 2 \left(\frac{1}{2}\right) x \cos(\angle AFC)
- We know cos⁡(∠AFC)=−cos⁡(∠AFE)\cos(\angle AFC) = -\cos(\angle AFE).
- By the Law of Cosines on △AFE\triangle AFE:
1=(12)2+2−2cos⁡(∠AFE) 1 = \left(\frac{1}{2}\right)^2 + 2 - \sqrt{2} \cos(\angle AFE)
- Solving for cos⁡(∠AFE)\cos(\angle AFE):
1=14+2−2cos⁡(∠AFE)  ⟹  cos⁡(∠AFE)=528 1 = \frac{1}{4} + 2 - \sqrt{2} \cos(\angle AFE) \implies \cos(\angle AFE) = \frac{5\sqrt{2}}{8}
- Thus, cos⁡(∠AFC)=−528\cos(\angle AFC) = -\frac{5\sqrt{2}}{8}.
- Plugging back in:
AC2=x2+14+52x8 AC^2 = x^2 + \frac{1}{4} + \frac{5\sqrt{2}x}{8}

7. Apply the Power of a Point theorem:
- By the Power of a Point theorem:
AC2=CF⋅CE  ⟹  x2+14+52x8=x(x+2) AC^2 = CF \cdot CE \implies x^2 + \frac{1}{4} + \frac{5\sqrt{2}x}{8} = x(x + \sqrt{2})
- Simplifying:
x2+14+52x8=x2+x2 x^2 + \frac{1}{4} + \frac{5\sqrt{2}x}{8} = x^2 + x\sqrt{2}
- Solving for xx:
32x8=14  ⟹  x=23=CF \frac{3\sqrt{2}x}{8} = \frac{1}{4} \implies x = \frac{\sqrt{2}}{3} = CF
- Therefore, CE=CF+FE=23+2=423CE = CF + FE = \frac{\sqrt{2}}{3} + \sqrt{2} = \frac{4\sqrt{2}}{3}.

The final answer is 423\boxed{\frac{4\sqrt{2}}{3}}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.