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Geometry Difficulty 7.4 National Olympiad, round 2 Prove it United States

In triangle ABCABC, points P,Q,RP, Q, R lie on sides BC,CA,ABBC, CA, AB, respectively. Let ωA,ωB,ωC\omega_A, \omega_B, \omega_C denote the circumcircles of triangles AQR,BRP,CPQAQR, BRP, CPQ, respectively. Given the fact that segment APAP intersects ωA,ωB,ωC\omega_A, \omega_B, \omega_C again at X,Y,ZX, Y, Z respectively, prove that YX/XZ=BP/PCYX/XZ = BP/PC.

(This problem was suggested by Zuming Feng.)

Figure 1

Solutions — 3

Solution 1

Assume that ωB\omega_B and ωC\omega_C intersect again at second point SS other than PP. If not, the degenerate case where ωB\omega_B and ωC\omega_C are tangent at PP can be dealt similarly. Because BPSRBPSR and CPSQCPSQ are cyclic, we have RSP=180PBR\angle RSP = 180^\circ - \angle PBR and PSQ=180QCP\angle PSQ = 180^\circ - \angle QCP. Hence, we obtain
QSR=360RSPPSQ=PBR+QCP=CBA+ACB=180BAC, \angle QSR = 360^\circ - \angle RSP - \angle PSQ = \angle PBR + \angle QCP = \angle CBA + \angle ACB = 180^\circ - \angle BAC,
from which it follows that ARSQARSQ is cyclic. This means that ωA,ωB\omega_A, \omega_B, and ωC\omega_C meet at SS. Note that this is the statement of Miquel's theorem.
Because BPSYBPSY is inscribed in ωB\omega_B, we find XYS=PYS=PBS\angle XYS = \angle PYS = \angle PBS. Because ARXSARXS is inscribed in ωA\omega_A, we find SXY=SXA=SRA\angle SXY = \angle SXA = \angle SRA. Because BPSRBPSR is inscribed in ωB\omega_B, we find SRA=SPB\angle SRA = \angle SPB. Thus, we have SXY=SRA=SPB\angle SXY = \angle SRA = \angle SPB. In triangles SYXSYX and SBPSBP, we have XYS=PBS\angle XYS = \angle PBS and SXY=SPB\angle SXY = \angle SPB. Therefore, triangles SYXSYX and SBPSBP are similar to each other which in particular implies
YXBP=SXSP. \frac{YX}{BP} = \frac{SX}{SP}.
In a similar manner, we can show that triangles SXZSXZ and SPCSPC are similar to each other and that
SXSP=XZPC. \frac{SX}{SP} = \frac{XZ}{PC}.
Combining the last two equations yields the desired result.

Solution 2

We consider the configuration shown in the diagram above. We can adjust the proof easily for other configurations. In particular, our proof uses directed angles modulo 180180^\circ.
Let line RYRY intersect ωA\omega_A again at TYT_Y (other than RR). Because BPYRBPYR is cyclic, TYYX=TYYP=RBP=ABP\angle T_Y YX = \angle T_Y YP = \angle RBP = \angle ABP. Because ARXTYARXT_Y is cyclic, XTYY=XAR=PAB\angle XT_Y Y = \angle XAR = \angle PAB. Hence triangles TYYXT_Y YX and ABPABP are similar to each other. In particular, we conclude that
YXTY=BPAandYXBP=XTYPA.(12) \angle YXT_Y = \angle BPA \quad \text{and} \quad \frac{YX}{BP} = \frac{XT_Y}{PA}. \qquad (12)
Likewise, if line QZQZ intersect ωA\omega_A again at TZT_Z (other than RR), we can show that triangles TZZXT_ZZX and ACPACP are similar to each other and that
TZXZ=APCandXTZPA=XZPC.(13) \angle T_Z XZ = \angle APC \quad \text{and} \quad \frac{XT_Z}{PA} = \frac{XZ}{PC}. \qquad (13)
In the light of the second equalities in (12) and (13), it suffices to show that TZ=TYT_Z = T_Y. On the other hand, the first equalities in (12) and (13) imply that X,TY,TZX, T_Y, T_Z lie on a line. But this line has only two intersections with ωA\omega_A with XX being one of them. Hence we must have TY=TZT_Y = T_Z, completing our proof.

Solution 3

We maintain the configuration and the notations of the second solution. Let T1T_1 denote the intersection of lines RYRY and QZQZ. Because BPYRBPYR and CPZQCPZQ are cyclic, we have T1YZ=T1YP=RBP=ABC\angle T_1YZ = \angle T_1YP = \angle RBP = \angle ABC and YZT1=YZQ=PCQ=BCA\angle YZT_1 = \angle YZQ = \angle PCQ = \angle BCA. Hence, triangles T1YZT_1YZ and ABCABC are similar to each other. In particular, we have ZT1R=ZT1Y=CAB=QAR\angle ZT_1R = \angle ZT_1Y = \angle CAB = \angle QAR, implying that AQRT1AQRT_1 is cyclic. Therefore, T1T_1 lies on ωA\omega_A and T1=TT_1 = T. Because ARXT1ARXT_1 is cyclic, XT1Y=XT1R=XAR=PAB\angle XT_1Y = \angle XT_1R = \angle XAR = \angle PAB, hence XX and PP are corresponding points in the similar triangles T1YZT_1YZ and ABCABC, from which the desired result follows.

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