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Geometry Difficulty 6.3 National Olympiad Prove it Ireland

Circles Ω1\Omega_1, centre QQ, and Ω2\Omega_2, centre RR, touch externally at BB. A third circle, Ω3\Omega_3, which contains Ω1\Omega_1 and Ω2\Omega_2, touches Ω1\Omega_1 and Ω2\Omega_2 at AA and CC, respectively. Point CC is joined to BB and the line BCBC is extended to meet Ω3\Omega_3 at DD.
Prove that QRQR and ADAD intersect on the circumference of Ω1\Omega_1.

Solutions — 5

Solution 1

Let MM be the centre of Ω3\Omega_3. Then, AA, QQ, MM are collinear as are CC, RR, MM. Let HH be the intersection point of the lines QRQR and ADAD.

Figure 1

Because BR=CR|BR| = |CR| and DM=CM|DM| = |CM|, we have MDC=MCD=RCB=RBC\angle MDC = \angle MCD = \angle RCB = \angle RBC, hence DMDM is parallel to BRBR. Using that RR, BB, QQ, HH are collinear and DM=AM|DM| = |AM| we now get AHQ=ADM=MAD=QAH\angle AHQ = \angle ADM = \angle MAD = \angle QAH hence triangle AHQAHQ is isosceles with QH=QA|QH| = |QA| and so HH is on Ω1\Omega_1.

Alternatively, we may consider HBA\triangle HBA, which has a right angle at AA. Because the centre, QQ, of Ω1\Omega_1 is on HBHB, this implies that HH must be on Ω1\Omega_1.

Solution 2

Draw tangents at AA and CC to Ω3\Omega_3 and let them intersect at TT. The radical axis of two circles that are tangent to each other is their common tangent. Therefore, TT is the radical centre of Ω1\Omega_1, Ω2\Omega_2 and Ω3\Omega_3, which implies that BTBT is the common tangent of Ω1\Omega_1 and Ω2\Omega_2. Hence, TBTB is perpendicular to QRQR. Let EE be the second intersection point of the line ABAB with Ω3\Omega_3, HH the intersection point of the lines QRQR and ADAD, and FF the intersection point of TBTB and DEDE. From TA=TB=TC|TA| = |TB| = |TC| we obtain
α:=TAC=TCAβ:=TAB=TBAγ:=TCB=TBC=FBD. \begin{align*} \alpha &:= \angle TAC = \angle TCA \\ \beta &:= \angle TAB = \angle TBA \\ \gamma &:= \angle TCB = \angle TBC = \angle FBD. \end{align*}
Considering the angle sum in ABC\triangle ABC we see that 180=BAC+ABC+BCA=(βα)+(β+γ)+(γα)=2(γ+βα)180^\circ = \angle BAC + \angle ABC + \angle BCA = (\beta - \alpha) + (\beta + \gamma) + (\gamma - \alpha) = 2(\gamma + \beta - \alpha), hence γ+βα=90\gamma + \beta - \alpha = 90^\circ.
Considering the external angle at BB in BCE\triangle BCE, we obtain BCE+AEC=ABC\angle BCE + \angle AEC = \angle ABC, i.e. BCE=ABCAEC=γ+βα=90\angle BCE = \angle ABC - \angle AEC = \gamma + \beta - \alpha = 90^\circ.
The Alternate Segment Theorem applied to circle Ω3\Omega_3 gives us
AEC=TAC=αADE=TAB=βDEC=TCB=γ. \begin{align*} \angle AEC &= \angle TAC = \alpha \\ \angle ADE &= \angle TAB = \beta \\ \angle DEC &= \angle TCB = \gamma. \end{align*}
Similarly, when we consider the external angle at BB in BAD\triangle BAD we can use ADC=AEC=α\angle ADC = \angle AEC = \alpha to obtain BAD=90\angle BAD = 90^\circ.
We can now conclude in two different ways. One way is to consider the triangles DFB\triangle DFB and DCE\triangle DCE which have an angle in common at DD. Because FBD=DEC=γ\angle FBD = \angle DEC = \gamma, we see that DFB=DCE=90\angle DFB = \angle DCE = 90^\circ. Hence TBTB is perpendicular to DEDE and so DEQRDE \parallel QR. This implies that
AHR=ADE=β=TAB, \angle AHR = \angle ADE = \beta = \angle TAB,
hence HH is on Ω1\Omega_1 by the converse of the Alternate Segment Theorem.

Alternatively, we may consider HBA\triangle HBA, which has a right angle at AA. Because the centre, QQ, of Ω1\Omega_1 is on HBHB, this implies that HH must be on Ω1\Omega_1.

Solution 3

We use the diagram and notations from Solution 2 and let MM be the centre of Ω3\Omega_3. Then the homothety of centre AA sending Ω1\Omega_1 to Ω3\Omega_3 sends QBQB to MEME and the homothety of centre CC sending Ω2\Omega_2 to Ω3\Omega_3 sends RBRB to MDMD. Since QQ, BB, RR are collinear, it follows that DD, MM, EE are collinear hence BAD=90\angle BAD = 90^\circ. We conclude as in Solution 2.

Solution 4

Extend AQAQ to the diameter ALAL of circle Ω1\Omega_1 and let KK be the second intersection point of ACAC with Ω1\Omega_1. Let MM be the centre of Ω3\Omega_3.

Figure 2

Consider the angle between ACAC and the common tangent to Ω1\Omega_1 and Ω3\Omega_3 at AA. The Alternate Segment Theorem for these circles then yields
ADC=ALK.(2) \angle ADC = \angle ALK. \qquad (2)
Note that QBD=RBC=BCR\angle QBD = \angle RBC = \angle BCR as well as QAB=QBA\angle QAB = \angle QBA. Because MAC=ACM\angle MAC = \angle ACM, we get
BCR+BCA=QBA+BAC.(3) \angle BCR + \angle BCA = \angle QBA + \angle BAC. \qquad (3)
Considering the external angle of triangle ABCABC at BB, we get
BAC+BCA=QBA+QBD.(4) \angle BAC + \angle BCA = \angle QBA + \angle QBD. \qquad (4)
Subtracting (3) from (4), we obtain now BAC=QBD\angle BAC = \angle QBD. Together with QAB=QBA\angle QAB = \angle QBA we now see that
LAK=QAB+BAC=QBA+QBD=ABD. \angle LAK = \angle QAB + \angle BAC = \angle QBA + \angle QBD = \angle ABD.
If we combine this with (2), we get
DAB=180ADCABD=180ALKLAK=AKL=90. \angle DAB = 180^\circ - \angle ADC - \angle ABD = 180^\circ - \angle ALK - \angle LAK = \angle AKL = 90^\circ.
If HH denotes the second intersection point of ADAD and Ω1\Omega_1, we have HAB=DAB=90\angle HAB = \angle DAB = 90^\circ, i.e. HBHB is a diameter of Ω1\Omega_1 and so HH is the intersection point of QRQR and ADAD.

Solution 5

Let HH be the intersection point of the lines QRQR and ADAD. Lines AQAQ and CRCR meet at the centre MM of circle Ω3\Omega_3.

Figure 3

From BRC\triangle BRC we see that QRM=BCR+RBC=2RBC=2HBD\angle QRM = \angle BCR + \angle RBC = 2\angle RBC = 2\angle HBD. The central angle AMC\angle AMC stands on the same arc of Ω3\Omega_3 as ADC\angle ADC, hence QMR=2HDB\angle QMR = 2\angle HDB. Considering external angles of triangles QMRQMR and HDBHDB we obtain
AQR=QMR+QRM=2HDB+2HBD=2AHB \angle AQR = \angle QMR + \angle QRM = 2\angle HDB + 2\angle HBD = 2\angle AHB
and this implies that HH is on Ω1\Omega_1.

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