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Geometry Difficulty 5.5 AIME, harder Prove it Belarus

Circles Γ1\Gamma_1 and Γ2\Gamma_2 meet at points XX and YY. A circle S1S_1 touches Γ1\Gamma_1 internally at AA and Γ2\Gamma_2 externally at BB. A circle S2S_2 touches Γ2\Gamma_2 internally at CC and Γ1\Gamma_1 externally at DD.

Prove that the points AA, BB, CC, DD are either collinear or concyclic.

Solution

Let O1O_1 be the center of Γ1\Gamma_1 and R1R_1 be its radius. Let O2O_2 be the center of Γ2\Gamma_2 and R2R_2 be its radius. Homothety with center at AA and with a positive coefficient transforms Γ1\Gamma_1 to ω1\omega_1. Homothety with center at BB and with a negative coefficient transforms ω1\omega_1 to Γ2\Gamma_2. Let ZZ be the center of the homothety with the negative coefficient which transforms Γ1\Gamma_1 to Γ2\Gamma_2. By the theorem of three homotheties, ZZ lies on the line ABAB. Similarly, ZZ lies on the line CDCD. It is evident that ZZ lies on the segment O1O2O_1O_2 and O1Z:ZO2=R1:R2O_1Z : ZO_2 = R_1 : R_2.

Figure 1

Figure 2

Show that ZBZA=ZCZDZB \cdot ZA = ZC \cdot ZD. Let EE be the intersection point (different from DD) of the line CDCD and Γ1\Gamma_1. The power of point ZZ with respect to Γ1\Gamma_1 is equal to R12O1Z2R_1^2 - O_1Z^2, on the other hand, it is equal to ZCZEZC \cdot ZE. From homothety's properties we have EZ=R1R2ZDEZ = \frac{R_1}{R_2} ZD. Then ZCZE=ZCR1R2ZD=R12O1Z2ZC \cdot ZE = ZC \cdot \frac{R_1}{R_2} ZD = R_1^2 - O_1Z^2, i.e. ZCZD=R2R1(R12O1Z2)ZC \cdot ZD = \frac{R_2}{R_1}(R_1^2 - O_1Z^2). Since O1Z=R1R1+R2O1O2O_1Z = \frac{R_1}{R_1+R_2} O_1O_2, we have
ZCZD=R2R1(R12R12O1O22(R1+R2)2)=R1R2(1O1O22(R1+R2)2). ZC \cdot ZD = \frac{R_2}{R_1} \left( R_1^2 - R_1^2 \frac{O_1 O_2^2}{(R_1 + R_2)^2} \right) = R_1 R_2 \left( 1 - \frac{O_1 O_2^2}{(R_1 + R_2)^2} \right).
Similarly, we obtain that ZBZA=R1R2(1O1O22(R1+R2)2)ZB \cdot ZA = R_1 R_2 \left( 1 - \frac{O_1 O_2^2}{(R_1 + R_2)^2} \right). Therefore, if AA, BB, CC, and DD do not lie on the same line, then they lie on the same circle.

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