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Geometry Difficulty 5.8 AIME, harder Prove it Estonia

Circles ω1\omega_1 and ω2\omega_2 touch at point KK. The line through the centres of the circles intersects the circle ω1\omega_1 once more at point AA. A line through the point AA intersects the circle ω1\omega_1 once more at point BB and the circle ω2\omega_2 at points CC and DD, where the points A,B,C,DA, B, C, D lie on the line in the order. Given that the line segments ABAB, BCBC and CDCD have equal lengths, find the ratio of the radii of the circles ω1\omega_1 and ω2\omega_2.

Solutions — 2

Solution 1

Let the radii of ω1\omega_1 and ω2\omega_2 be r1r_1 and r2r_2 respectively. Let EE be the second intersection of AKAK and ω2\omega_2 (Fig. 40). As AKAK and KEKE are diameters of ω1\omega_1 and ω2\omega_2 respectively, the angles ABKABK and KDEKDE must be right angles. In triangle AKCAKC, the segment KBKB is both a median and an altitude, thus KA=KCKA = KC and KAC=ACK\angle KAC = \angle ACK. As CDEKCDEK is cyclic, we have
EAD=KAC=ACK=DEA. \angle EAD = \angle KAC = \angle ACK = \angle DEA.
Thus ADEADE is isosceles, also KBAKBA and KDEKDE are similar. Therefore
r1r2=AKKE=ABDE=ABAD=13. \frac{r_1}{r_2} = \frac{AK}{KE} = \frac{AB}{DE} = \frac{AB}{AD} = \frac{1}{3}.

Figure 1
Fig. 40

Solution 2

Let the radii of ω1\omega_1 and ω2\omega_2 be r1r_1 and r2r_2 respectively. Let PP be the midpoint of CDCD and QQ the center of ω2\omega_2 (Fig. 41). Then PQPQ is perpendicular to CDCD, as it connects the midpoint of the chord CDCD of ω2\omega_2 and the center of ω2\omega_2. Thus APQ=CPQ=90\angle APQ = \angle CPQ = 90^\circ. As AKAK is a diameter of ω1\omega_1, we have ABK=90\angle ABK = 90^\circ. Therefore BKPQBK \parallel PQ. Together with AB=BC=CDAB = BC = CD, we get AQAK=APAB=2.5\frac{AQ}{AK} = \frac{AP}{AB} = 2.5 or 2AQ=5AK2AQ = 5AK. From here 2(2r1+r2)=52r12(2r_1 + r_2) = 5 \cdot 2r_1, from which r1r2=13\frac{r_1}{r_2} = \frac{1}{3}.

Figure 2
Fig. 41

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