Circles ω1 and ω2 touch at point K. The line through the centres of the circles intersects the circle ω1 once more at point A. A line through the point A intersects the circle ω1 once more at point B and the circle ω2 at points C and D, where the points A,B,C,D lie on the line in the order. Given that the line segments AB, BC and CD have equal lengths, find the ratio of the radii of the circles ω1 and ω2.
Solutions — 2
Solution 1
Let the radii of ω1 and ω2 be r1 and r2 respectively. Let E be the second intersection of AK and ω2 (Fig. 40). As AK and KE are diameters of ω1 and ω2 respectively, the angles ABK and KDE must be right angles. In triangle AKC, the segment KB is both a median and an altitude, thus KA=KC and ∠KAC=∠ACK. As CDEK is cyclic, we have ∠EAD=∠KAC=∠ACK=∠DEA. Thus ADE is isosceles, also KBA and KDE are similar. Therefore r2r1=KEAK=DEAB=ADAB=31.
Fig. 40
Solution 2
Let the radii of ω1 and ω2 be r1 and r2 respectively. Let P be the midpoint of CD and Q the center of ω2 (Fig. 41). Then PQ is perpendicular to CD, as it connects the midpoint of the chord CD of ω2 and the center of ω2. Thus ∠APQ=∠CPQ=90∘. As AK is a diameter of ω1, we have ∠ABK=90∘. Therefore BK∥PQ. Together with AB=BC=CD, we get AKAQ=ABAP=2.5 or 2AQ=5AK. From here 2(2r1+r2)=5⋅2r1, from which r2r1=31.
Fig. 41
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