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Geometry Difficulty 7.5 National Olympiad, round 2 Prove it Philippines

Problem:

In right triangle ABCA B C, ACB=90\angle A C B = 90^{\circ} and tanA>2\tan A > \sqrt{2}. MM is the midpoint of ABA B, PP is the foot of the altitude from CC, and NN is the midpoint of CPC P. Line ABA B meets the circumcircle of CNBC N B again at QQ. RR lies on line BCB C such that QRQ R and CPC P are parallel, SS lies on ray CAC A past AA such that BR=RSB R = R S, and VV lies on segment SPS P such that AV=VPA V = V P. Line SPS P meets the circumcircle of CPBC P B again at TT. WW lies on ray VAV A past AA such that 2AW=ST2 A W = S T, and OO is the circumcenter of SPMS P M. Prove that lines OMO M and BWB W are perpendicular.

Solution

Solution:

Refer to the figure shown below.

Figure 1

Since CPC P is the CC-altitude of ABCA B C, triangles ACPA C P, ABCA B C and CBPC B P are similar, so AP=AC2ABA P = \frac{A C^{2}}{A B} and BP=BC2ABB P = \frac{B C^{2}}{A B}. As CNQBC N Q B is cyclic, we have PNQ=CBP=ACP\angle P N Q = \angle C B P = \angle A C P so triangles ACPA C P and QNPQ N P are similar. With NN being the midpoint of PCP C, we have APPQ=PCPN=2\frac{A P}{P Q} = \frac{P C}{P N} = 2, which gives PQ=12APP Q = \frac{1}{2} A P and BQ=APAQ=AB32APB Q = A P - A Q = A B - \frac{3}{2} A P.

We now claim that SC2=2ABPMS C^{2} = 2 A B \cdot P M. Indeed, since QRQ R and CPC P are parallel, we have BQBP=BRBC\frac{B Q}{B P} = \frac{B R}{B C} by Thales' Theorem. Thus,
RS=BR=BCBQBP=BC(AB3AC22AB)BC2AB=2AB23AC22BC R S = B R = \frac{B C \cdot B Q}{B P} = \frac{B C\left(A B - \frac{3 A C^{2}}{2 A B}\right)}{\frac{B C^{2}}{A B}} = \frac{2 A B^{2} - 3 A C^{2}}{2 B C}
Applying Pythagorean theorem on triangle SCRS C R, we see that
SC2=BR2RC2=BC(2BRBC)=BC(2AB23AC2BCBC)=2BC2+2CA23CA2BC2=BC2CA2 \begin{aligned} S C^{2} & = B R^{2} - R C^{2} = B C (2 B R - B C) = B C\left(\frac{2 A B^{2} - 3 A C^{2}}{B C} - B C\right) \\ & = 2 B C^{2} + 2 C A^{2} - 3 C A^{2} - B C^{2} = B C^{2} - C A^{2} \end{aligned}
Note that tanA>2\tan A > \sqrt{2} implies that SS is indeed on ray CAC A past AA. On the other hand, with MM being the midpoint of ABA B, we compute
PM=AMAP=AB2CA2AB=AB22CA22AB=BC2CA22AB=SC22AB P M = A M - A P = \frac{A B}{2} - \frac{C A^{2}}{A B} = \frac{A B^{2} - 2 C A^{2}}{2 A B} = \frac{B C^{2} - C A^{2}}{2 A B} = \frac{S C^{2}}{2 A B}
which proves the desired claim.

Observe that the circumcircle of CPBC P B is tangent to SCS C at CC, so the Power of the Point gives SC2=SPSTS C^{2} = S P \cdot S T. It follows from the above claim that
SPST=2ABPMSPPM=2ABST=2AB2AW=ABAW S P \cdot S T = 2 A B \cdot P M \Longleftrightarrow \frac{S P}{P M} = \frac{2 A B}{S T} = \frac{2 A B}{2 A W} = \frac{A B}{A W}
Since AV=PVA V = P V, we have WAB=180VAP=180VPA=MPS\angle W A B = 180^{\circ} - \angle V A P = 180^{\circ} - \angle V P A = \angle M P S, so triangles WABW A B and MPSM P S are similar. Thus, we see that PSM=ABW\angle P S M = \angle A B W and with OP=OMO P = O M, we arrive at
ABW+PMO=PSM+PMO=12(POM+2PMO)=12180=90 \angle A B W + \angle P M O = \angle P S M + \angle P M O = \frac{1}{2}(\angle P O M + 2 \angle P M O) = \frac{1}{2} \cdot 180^{\circ} = 90^{\circ}
Hence, we get OMBWO M \perp B W, which completes the proof.

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