In right triangle ABC, ∠ACB=90∘ and tanA>2. M is the midpoint of AB, P is the foot of the altitude from C, and N is the midpoint of CP. Line AB meets the circumcircle of CNB again at Q. R lies on line BC such that QR and CP are parallel, S lies on ray CA past A such that BR=RS, and V lies on segment SP such that AV=VP. Line SP meets the circumcircle of CPB again at T. W lies on ray VA past A such that 2AW=ST, and O is the circumcenter of SPM. Prove that lines OM and BW are perpendicular.
Solution
Solution:
Refer to the figure shown below.
Since CP is the C-altitude of ABC, triangles ACP, ABC and CBP are similar, so AP=ABAC2 and BP=ABBC2. As CNQB is cyclic, we have ∠PNQ=∠CBP=∠ACP so triangles ACP and QNP are similar. With N being the midpoint of PC, we have PQAP=PNPC=2, which gives PQ=21AP and BQ=AP−AQ=AB−23AP.
We now claim that SC2=2AB⋅PM. Indeed, since QR and CP are parallel, we have BPBQ=BCBR by Thales' Theorem. Thus, RS=BR=BPBC⋅BQ=ABBC2BC(AB−2AB3AC2)=2BC2AB2−3AC2 Applying Pythagorean theorem on triangle SCR, we see that SC2=BR2−RC2=BC(2BR−BC)=BC(BC2AB2−3AC2−BC)=2BC2+2CA2−3CA2−BC2=BC2−CA2 Note that tanA>2 implies that S is indeed on ray CA past A. On the other hand, with M being the midpoint of AB, we compute PM=AM−AP=2AB−ABCA2=2ABAB2−2CA2=2ABBC2−CA2=2ABSC2 which proves the desired claim.
Observe that the circumcircle of CPB is tangent to SC at C, so the Power of the Point gives SC2=SP⋅ST. It follows from the above claim that SP⋅ST=2AB⋅PM⟺PMSP=ST2AB=2AW2AB=AWAB Since AV=PV, we have ∠WAB=180∘−∠VAP=180∘−∠VPA=∠MPS, so triangles WAB and MPS are similar. Thus, we see that ∠PSM=∠ABW and with OP=OM, we arrive at ∠ABW+∠PMO=∠PSM+∠PMO=21(∠POM+2∠PMO)=21⋅180∘=90∘ Hence, we get OM⊥BW, which completes the proof.
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