Maths Olympiad Prep

Library / /57 of 73

Geometry Difficulty 8.5 Shortlist Prove it Turkey

Let ABCABC be a triangle with circumcircle ω\omega and let ωA\omega_A be a circle drawn outside ABCABC and tangent to side BCBC at A1A_1 and tangent to ω\omega at A2A_2. Let the circles ωB\omega_B and ωC\omega_C and the points B1,B2,C1,C2B_1, B_2, C_1, C_2 are defined similarly. Prove that if the lines AA1,BB1,CC1AA_1, BB_1, CC_1 are concurrent, then the lines AA2,BB2,CC2AA_2, BB_2, CC_2 are also concurrent.

Solution

Let A1A2A_1A_2 intersect ω\omega at A3A_3. Consider the common tangent of circles ω\omega and ωA\omega_A passing through A2A_2 and let UU be a point on this common tangent which is at the same side of the line A1A2A_1A_2 with respect to BB. Then by tangency, we have A1A2U=BA1A2\angle A_1A_2U = \angle BA_1A_2 and BA2U=BA3A2\angle BA_2U = \angle BA_3A_2. Therefore
CA2A3=A3BC=BA1A2BA3A2=A1A2UBA2U=BA2A1=A3CB \angle CA_2A_3 = \angle A_3BC = \angle BA_1A_2 - \angle BA_3A_2 = \angle A_1A_2U - \angle BA_2U = \angle BA_2A_1 = \angle A_3CB
which means that A3B=A3CA_3B = A_3C. By the sine law, we obtain that
A1BA1C=A3BA3CsinBA3A2sinCA3A2=sinBA3A2sinCA3A2 \frac{A_1B}{A_1C} = \frac{A_3B}{A_3C} \cdot \frac{\sin \angle BA_3A_2}{\sin \angle CA_3A_2} = \frac{\sin \angle BA_3A_2}{\sin \angle CA_3A_2}

and since BAA2=BA3A2\angle BAA_2 = \angle BA_3A_2 and CAA2=CA3A2\angle CAA_2 = \angle CA_3A_2, we obtain that
A1BA1C=sinBAA2sinCAA2 \frac{A_1B}{A_1C} = \frac{\sin \angle BAA_2}{\sin \angle CAA_2}
Similarly, we get
B1CB1A=sinCBB2sinABB2andC1AC1B=sinACC2sinBCC2 \frac{B_1C}{B_1A} = \frac{\sin \angle CBB_2}{\sin \angle ABB_2} \quad \text{and} \quad \frac{C_1A}{C_1B} = \frac{\sin \angle ACC_2}{\sin \angle BCC_2}
Therefore, we have
A1BA1CB1CB1AC1AC1B=sinBAA2sinCAA2sinCBB2sinABB2sinACC2sinBCC2 \frac{A_1B}{A_1C} \cdot \frac{B_1C}{B_1A} \cdot \frac{C_1A}{C_1B} = \frac{\sin \angle BAA_2}{\sin \angle CAA_2} \cdot \frac{\sin \angle CBB_2}{\sin \angle ABB_2} \cdot \frac{\sin \angle ACC_2}{\sin \angle BCC_2}

Figure 1
Since AA1,BB1,CC1AA_1, BB_1, CC_1 are concurrent, by Ceva Theorem, we have
A1BA1CB1CB1AC1AC1B=1 \frac{A_1B}{A_1C} \cdot \frac{B_1C}{B_1A} \cdot \frac{C_1A}{C_1B} = 1
and hence
sinBAA2sinCAA2sinCBB2sinABB2sinACC2sinBCC2=1 \frac{\sin \angle BAA_2}{\sin \angle CAA_2} \cdot \frac{\sin \angle CBB_2}{\sin \angle ABB_2} \cdot \frac{\sin \angle ACC_2}{\sin \angle BCC_2} = 1
which means that the lines AA2,BB2,CC2AA_2, BB_2, CC_2 are also concurrent by the Converse Trigonometric Ceva Theorem.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.