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Geometry Difficulty 6.3 National olympiad Prove it Estonia

Inside a circle cc there are circles c1c_1, c2c_2 and c3c_3 which are tangent to cc at points AA, BB and CC correspondingly, which are all different. Circles c2c_2 and c3c_3 have a common point KK in the segment BCBC, circles c3c_3 and c1c_1 have a common point LL in the segment CACA, and circles c1c_1 and c2c_2 have a common point MM in the segment ABAB. Prove that the circles c1c_1, c2c_2 and c3c_3 intersect in the center of the circle cc.

Solution

Take a point XX on the common tangent to the circles c1c_1 and cc which lies on the other side of the line ABAB from the point CC. Then ALM=XAM=XAB=ACB\angle ALM = \angle XAM = \angle XAB = \angle ACB (Fig. 4). Consequently MLBCML \parallel BC. Similarly KMCAKM \parallel CA and LKABLK \parallel AB. If AMAB=λ\frac{|AM|}{|AB|} = \lambda, then
BKBC=BMBA=1λandCLCA=CKCB=1(1λ)=λ,whenceλ=AMAB=ALAC=1λ.Henceλ=12, therefore the triangles AML,MBK and LKC are all similar \frac{|BK|}{|BC|} = \frac{|BM|}{|BA|} = 1 - \lambda \quad \text{and} \quad \frac{|CL|}{|CA|} = \frac{|CK|}{|CB|} = 1 - (1 - \lambda) = \lambda, \quad \text{whence} \quad \lambda = \frac{|AM|}{|AB|} = \frac{|AL|}{|AC|} = 1 - \lambda. \quad \text{Hence} \quad \lambda = \frac{1}{2}, \text{ therefore the triangles } AML, MBK \text{ and } LKC \text{ are all similar}
to ABCABC with the factor 12\frac{1}{2}. Thus the radii of their circumcircles c1c_1, c2c_2 and c3c_3 are equal to half of the radius of the circumcircle cc of the triangle ABCABC. Since the circles cc and c1c_1 are tangent, the diameter of c1c_1 and the radius of cc, both drawn from the tangent point AA, coincide. Hence the circle c1c_1 goes through the center of the circle cc; similarly the circles c2c_2 and c3c_3 go through the center of the circle cc.

Figure 1
Figure 4

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