Maths Olympiad Prep

Library / /106 of 155

Geometry Difficulty 6.5 National olympiad Prove it Saudi Arabia

Let ABCABC be a non isosceles triangle with circumcircle (O)(O) and incircle (I)(I). Denote (O1)(O_{1}) as the circle that is externally tangent to (O)(O) at AA', and also tangent to the lines ABAB, ACAC at AbA_b, AcA_c respectively. Define the circles (O2)(O_{2}), (O3)(O_{3}) and the points BB', CC', BcB_c, BaB_a, CaC_a, CbC_b similarly.

1. Denote JJ as the radical center of (O1)(O_{1}), (O2)(O_{2}), (O3)(O_{3}) and suppose that JAJA' intersects (O1)(O_{1}) at the second point XX, JBJB' intersects (O2)(O_{2}) at the second point YY, JCJC' intersects (O3)(O_{3}) at the second point ZZ. Prove that the circle (XYZ)(XYZ) is tangent to (O1)(O_{1}), (O2)(O_{2}), (O3)(O_{3}).

2. Prove that AAAA', BBBB', CCCC' are concurrent at the point MM and the three points II, MM, OO are collinear.

Solution

1) Consider the inversion with center JJ and ratio equal to the power of JJ to the three circles (O1)(O_{1}), (O2)(O_{2}), (O3)(O_{3}) as a function ff.
It is easy to see that
f((O1))=(O1),f((O2))=(O2),f((O3))=(O3). f\left((O_{1})\right) = (O_{1}), \quad f\left((O_{2})\right) = (O_{2}), \quad f\left((O_{3})\right) = (O_{3}) .
On the other hand, f(X)=A1f(X) = A_1, f(Y)=B1f(Y) = B_1, f(Z)=C1f(Z) = C_1, thus
f((XYZ))=(A1B1C1)=(O). f((XYZ)) = (A_1B_1C_1) = (O) .
Because (O)(O) is tangent to all three circles (O1)(O_{1}), (O2)(O_{2}), (O3)(O_{3}), then (XYZ)(XYZ) is also tangent to (O1)(O_{1}), (O2)(O_{2}), (O3)(O_{3}), based on the property of inversion.

Figure 1

2) We will prove that AAAA', BBBB', CCCC' are concurrent at the homothety center of (O)(O) and (I)(I). Indeed,
Suppose that AAAA' intersects IOIO at MM. It is easy to see that AA, II, O1O_1 are collinear and OO, O1O_1, AA' are also collinear. Denote rAr_A as the radius of (O1)(O_1).

Figure 2

From the ratio of radii, we can see that:
AIAO1=rrA and AOAO1=RrA. \frac{\overline{AI}}{\overline{AO_1}} = \frac{r}{r_A} \text{ and } \frac{\overline{A'O}}{\overline{A'O_1}} = -\frac{R}{r_A} .
By applying Menelaus' theorem, we have
AIAO1AO1AOMOMI=1MOMI=AO1AIAOAO1=rAr(RrA)=Rr. \frac{\overline{AI}}{\overline{AO_1}} \cdot \frac{\overline{A'O_1}}{\overline{A'O}} \cdot \frac{\overline{MO}}{\overline{MI}} = 1 \Leftrightarrow \frac{\overline{MO}}{\overline{MI}} = \frac{\overline{AO_1}}{\overline{AI}} \cdot \frac{\overline{A'O}}{\overline{A'O_1}} = \frac{r_A}{r} \left(-\frac{R}{r_A}\right) = -\frac{R}{r} .
Hence the line AAAA' passes through the point MM that internally divides the segment OIOI with ratio Rr\frac{R}{r}. Similarly for BBBB', CCCC'. Therefore, AAAA', BBBB', CCCC' are concurrent at a point MM belonging to the segment IOIO. \square

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 and solution reproduced as published; topic and difficulty added by this site.