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Geometry Difficulty 6.9 National olympiad Prove it Mongolia

Points BB, DD, EE, FF, CC lie on a line. Let AA be a point outside the line and let ω1,ω2,ω3,ω4\omega_1, \omega_2, \omega_3, \omega_4 be incircles of the triangles ABDABD, ADEADE, AEFAEF, AFCAFC respectively. Let p1,p2,p3,p4p_1, p_2, p_3, p_4 be outwardly inscribed circles of the triangles ABDABD, ADEADE, AEFAEF, AFCAFC at the vertex AA. Prove that if circles ω1,ω2,ω3,ω4\omega_1, \omega_2, \omega_3, \omega_4 have common tangent different from BCBC then circles p1,p2,p3,p4p_1, p_2, p_3, p_4 also have common tangent different from BCBC.

Solution

We need following fact.

Lemma: Given a point DD on side BCBC of the triangle ABCABC. Let ω1(O1,r1),ω2(O2,r2)\omega_1(O_1, r_1), \omega_2(O_2, r_2) be incircles of triangles ACDACD, ADBADB and ρ1(O3,r3),ρ2(O4,r4)\rho_1(O_3, r_3), \rho_2(O_4, r_4) be circles inscribed in these triangles externally, which tangent at an internal point of BCBC. Then lines O1O2O_1O_2, O3O4O_3O_4, BCBC pass through a point.

Figure 1

Proof of lemma: Denote BCO3O4=XBC \cap O_3O_4 = X and let's prove that XO1O2X \in O_1O_2. From the well known property that a line passing centers of circles inscribed in an angle is bisector of the angle, it follows A=(O1O3)(O2O4)A = (O_1O_3) \cap (O_2O_4), D=(O1O4)(O2O3)D = (O_1O_4) \cap (O_2O_3). Since the line O3O2O_3O_2 intersects sides of the triangle AO1O4AO_1O_4,

by Menelaus theorem we get 1=O3O1O3ADO4DO1O2AO2O4=O3AO1AO3ADO4DO1O2AO4AO2A \text{by Menelaus theorem we get } 1 = \frac{O_3O_1}{O_3A} \cdot \frac{DO_4}{DO_1} \cdot \frac{O_2A}{O_2O_4} = \frac{O_3A - O_1A}{O_3A} \cdot \frac{DO_4}{DO_1} \cdot \frac{O_2A}{O_4A - O_2A}

=(1O1AO3A)DO4DO1(1O4AO2A1)=(1r1r3)(r4r1)(1r4r21) = \left(1 - \frac{O_1A}{O_3A}\right) \cdot \frac{DO_4}{DO_1} \cdot \left(\frac{1}{\frac{O_4A}{O_2A} - 1}\right) = \left(1 - \frac{r_1}{r_3}\right) \left(\frac{r_4}{r_1}\right) \left(\frac{1}{\frac{r_4}{r_2} - 1}\right)

=(r3r1)r3r4r1r2r4r2=(r3r1)r4r2r3r1(r4r2)() = \frac{(r_3 - r_1)}{r_3} \cdot \frac{r_4}{r_1} \cdot \frac{r_2}{r_4 - r_2} = \frac{(r_3 - r_1) r_4 r_2}{r_3 r_1 (r_4 - r_2)} \quad (*)

To prove X(O1O2)X \in (O_1O_2) is equivalent to proving by Menelaus theorem:

XO3XO4O1AO1O3O2O4O2A=1. \frac{XO_3}{XO_4} \cdot \frac{O_1A}{O_1O_3} \cdot \frac{O_2O_4}{O_2A} = 1.

Furthermore,

XO3XO4=r3r4,O1AO1O3=O1AO3AAO1=1O3AO1A1=1r3r11=r1r3r1, \frac{XO_3}{XO_4} = \frac{r_3}{r_4}, \quad \frac{O_1A}{O_1O_3} = \frac{O_1A}{O_3A - AO_1} = \frac{1}{\frac{O_3A}{O_1A} - 1} = \frac{1}{\frac{r_3}{r_1} - 1} = \frac{r_1}{r_3 - r_1},

O2O4O2A=AO4AO2O2A=AO4AO21=r4r21=r4r2r2 \frac{O_2O_4}{O_2A} = \frac{AO_4 - AO_2}{O_2A} = \frac{AO_4}{AO_2} - 1 = \frac{r_4}{r_2} - 1 = \frac{r_4 - r_2}{r_2}

and by ()(*),

XO3XO4O1AO1O3O2O4O2A=r3r4r1r3r1r4r2r2=1. \frac{XO_3}{XO_4} \cdot \frac{O_1A}{O_1O_3} \cdot \frac{O_2O_4}{O_2A} = \frac{r_3}{r_4} \cdot \frac{r_1}{r_3 - r_1} \cdot \frac{r_4 - r_2}{r_2} = 1.

This proves that XO1O2X \in O_1O_2 and points O1O2O_1O_2, O3O4O_3O_4, BCBC pass through a point.

Now solve the problem. Let ll be common tangent to circles ω1,ω2,ω3,ω4\omega_1, \omega_2, \omega_3, \omega_4 which is different from BCBC. Denote lBC=Xl \cap BC = X. Centers of circles ω1,ω2,ω3,ω4\omega_1, \omega_2, \omega_3, \omega_4 lie on the bisector of angle formed by ll, BCBC and the point XX lies on this bisector too.

Using the lemma for triangles ABEABE, ADFADF, AECAEC we conclude that centers of circles ρ1,ρ2,ρ3,ρ4\rho_1, \rho_2, \rho_3, \rho_4 and the point XX lie on a line. Reflecting BCBC about this line we get a line which is common tangent to circles ρ1,ρ2,ρ3,ρ4\rho_1, \rho_2, \rho_3, \rho_4. Thus we have the desired result.

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