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Geometry Difficulty 6.8 National Olympiad Prove it Turkey

The circle ω1\omega_1 with diameter [AB][AB] and the circle ω2\omega_2 with center AA intersect at points CC and DD. Let EE be a point on the circle ω2\omega_2, which is outside ω1\omega_1 and at the same side with CC with respect to the line ABAB. Let the second point of intersection of the line BEBE with ω2\omega_2 be FF. Suppose that a point Kω1K \in \omega_1 is on the same side with AA with respect to the diameter of ω1\omega_1 passing through CC and 2CKAC=CEAB2 \cdot CK \cdot AC = CE \cdot AB. Let the second point of intersection of the line KFKF with ω1\omega_1 be LL. Show that a point symmetric to DD with respect to the line BEBE lies on the circumcircle of the triangle LFCLFC.

Solution

Figure 1

Let DD' be the point symmetric to DD with respect to the line BEBE. 2CKAC=CEAB2 \cdot CK \cdot AC = CE \cdot AB implies CK/CE=R1/R2CK/CE = R_1/R_2 where R1R_1 and R2R_2 are radii of the circles ω1\omega_1 and ω2\omega_2, respectively. By the sine law, 2sinCLF=CK/R1=CE/R2=2sinCFE2 \cdot \sin \angle CLF = CK/R_1 = CE/R_2 = 2 \cdot \sin \angle CFE, and hence CLF=CFE\angle CLF = \angle CFE since the sum of angles is less than 180180^\circ.

Let XX be the second intersection point of the line BEBE with ω1\omega_1. Since [AB][AB] is a diameter of ω1\omega_1 and AC=ADAC = AD, the smaller arcs BCBC and BDBD of ω1\omega_1 are equal and hence DXB=CXB\angle DXB = \angle CXB. Since DXB=DXB\angle D'XB = \angle DXB, we get that the points X,C,DX, C, D' are collinear. Since AXB=90\angle AXB = 90^\circ, we get EX=FXEX = FX and since ACB=90\angle ACB = 90^\circ, the line BCBC is tangent to ω2\omega_2 and hence FCB=BEC\angle FCB = \angle BEC. On the other hand, DCF=DEF\angle DCF = \angle DEF, therefore DEC=DCB=CXB=DXB\angle DEC = \angle DCB = \angle CXB = \angle DXB and hence ECX=DEX\angle ECX = \angle DEX and

CEX=EDX\angle CEX = \angle EDX.

Therefore the triangles ECXECX and DEXDEX are similar and it follows that XCXD=XCXD=EX2=FX2XC \cdot XD' = XC \cdot XD = EX^2 = FX^2. Since CLF=CFE\angle CLF = \angle CFE, the line XFXF is tangent to the circumcircle of triangle CLFCLF and since XCXD=FX2XC \cdot XD' = FX^2, the point DD' lies on this circle. Done.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.