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Geometry Difficulty 8.7 Shortlist Prove it Taiwan

Let ABCDABCD be a cyclic quadrilateral with circumcircle ω\omega and circumradius rr. The diagonals ACAC and BDBD intersect at PP. Suppose that AD=DPAD = DP. Let SS be the foot of the perpendicular from PP to the line ABAB. Point QQ lies on line SPSP such that PQ=rPQ = r and S,P,QS, P, Q lie on the line in that order. Let the line perpendicular to CQCQ from AA intersect the line perpendicular to DQDQ from BB at EE. Prove that EE lies on ω\omega.

Solutions — 2

Solution 1

First observe that
DPA=BPC=CBP=CBD=CAD=PAD \angle DPA = \angle BPC = \angle CBP = \angle CBD = \angle CAD = \angle PAD
so DP=DADP = DA. Thus there is a symmetry in the problem statement swapping (A,D)(B,C)(A, D) \leftrightarrow (B, C). Let OO be the centre of ω\omega and let EE be the reflection of PP in CDCD which, by
CED=DPC=180CPB=180PBC=180DBC \angle CED = \angle DPC = 180^\circ - \angle CPB = 180^\circ - \angle PBC = 180^\circ - \angle DBC
lies on ω\omega. We claim the two lines concur at EE. By the symmetry noted above, it suffices to prove that BEDQBE \perp DQ and then AECQAE \perp CQ will follow by symmetry. We have AO=PQ,AD=DPAO = PQ, AD = DP and
DAO=90ABD=DPQ. \angle DAO = 90^\circ - \angle ABD = \angle DPQ.
Hence AODPQD\triangle AOD \cong \triangle PQD. Thus
QDB+DBE=ODA+DAE=ODA+AED=90 \angle QDB + \angle DBE = \angle ODA + \angle DAE = \angle ODA + \angle AED = 90^\circ
giving BEDQBE \perp DQ as required.

Solution 2

As in Solution 1, we prove that DA=DPDA = DP and note the symmetry in the problem statement swapping (A,D)(B,C)(A, D) \leftrightarrow (B, C).
Let Γ\Gamma be the circumcircle of PCD\triangle PCD. Since DP=DADP = DA and ACD=PCD\angle ACD = \angle PCD, the radius of Γ\Gamma is equal to that of ω\omega. We have that
DPQBPS=90ABD=90PCD. \angle DPQ \angle BPS = 90^\circ - \angle ABD = 90^\circ - \angle PCD.
This, combined with PQPQ being equal to the common circumradius of Γ\Gamma and ω\omega, means that QQ is the circumcentre of Γ\Gamma. Let the perpendiculars to CQCQ, DQDQ from AA, BB intersect at EE then we have
EAC=90ACQ=90QPC=90SPA=CABEAB=2PAB \angle EAC = 90^\circ - \angle ACQ = 90^\circ - \angle QPC = 90^\circ - \angle SPA = \angle CAB \Rightarrow \angle EAB = 2\angle PAB
DBE=90QDP=90DPQ=90BPS=ABDABE=2ABP. \angle DBE = 90^\circ - \angle QDP = 90^\circ - \angle DPQ = 90^\circ - \angle BPS = \angle ABD \Rightarrow \angle ABE = 2\angle ABP.
Combining these
BEA=1802(PAB+ABP)=1802APD=BDA \angle BEA = 180^\circ - 2(\angle PAB + \angle ABP) = 180^\circ - 2\angle APD = \angle BDA
which gives that EE lies on ω\omega.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.