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Question Number 112266 by Aina Samuel Temidayo last updated on 07/Sep/20
Let Ω denote the circumcircle of ABC.  The tangent to Ω at A meets BC at X.  Let the angle bisectors of ∠AXB meet  AC and AB at E and F  respectively. D is the foot of the angle  bisector from ∠BAC on BC. Let AD  intersect EF at K and Ω again at  L(other than A). Prove that AEDF is  a rhombus and further prove that the  circle defined by triangle KLX passes  through the midpoint of line segment  BC.
LetΩdenotethecircumcircleofABC.ThetangenttoΩatAmeetsBCatX.LettheanglebisectorsofAXBmeetACandABatEandFrespectively.DisthefootoftheanglebisectorfromBAConBC.LetADintersectEFatKandΩagainatL(otherthanA).ProvethatAEDFisarhombusandfurtherprovethatthecircledefinedbytriangleKLXpassesthroughthemidpointoflinesegmentBC.
Answered by 1549442205PVT last updated on 07/Sep/20
Commented by 1549442205PVT last updated on 07/Sep/20
i)Denote by H the midpoint of BC.Since  AL is the bisector ray of the angle A^(�) ,  L is middle point of the smaller arc BC  Hence,three points O,H,L colinear.  and OHX^(�) =90°(property of the diameter)  On the other hands,OAX^(�) =90°(since  XA is the tangent at A of the circle(O))  We infer the quadrilateral AXHO is  inscribed)⇒AXH^(�) +AOH^(�) =180°(1).On the  other hands,the triangle OAL is isosceles  at O,so OAL^(�) =OLA^(�) =α.Moreover,  in the triangle OAL we have   OAL^(�) +OLA^(�) +AOH^(�) =180° infer  2α+AOH^(�) =180° (2).From (1)(2)we get  AXH^(�) =2α.From that since AXE^(�) =  (1/2)AXH^(�)  we infer OAL^(�) =AXE^(�) =α.  This gives us that KAX^(�) +AXE^(�) =  KAX^(�) +OAL^(�) =OAX^(�) =90°  Thus,KD⊥EF.Consider two teiangles  XAC and XBA have the common angle  AXC^(�) ,ACB^(�) =BAX^(�) .It follows that  ΔAXC∽ΔBXA(a.a)⇒((AX)/(BX))=((AC)/(AB))(3)  .On the other hands,since AD is the   bisector of the angle A^(�)  we have  ((BD)/(DC))=((AB)/(AC))(4).Since XF is the bisector  of the AXB^(�) ,((BF)/(FA))=((BX)/(AX)) (5).From (3)(4)  (5)we obtain ((BD)/(DC))=((BF)/(FA))⇒DF//AC.  Similarly,we have DE//AB  ,so  quadrilateral AEDF is a parallelogram  that has two mutual perpendicular   diagonals.That shows AEDF is a rhombus(q.e.d)  ii)By above proof we have O,H,L are  colinear (H−midpoint of BC) and  OLA^(�) =(1/2)AXH^(�) =KXH^(�) =α.Therefore,  quadrilateral HKXL is inscribed which  shows that the circumcircle of triangle  KXL go through the midpoint H of BC  (q.e.d)
i)DenotebyHthemidpointofBC.SinceAListhebisectorrayoftheangleA^,LismiddlepointofthesmallerarcBCHence,threepointsO,H,Lcolinear.andOHX^=90°(propertyofthediameter)Ontheotherhands,OAX^=90°(sinceXAisthetangentatAofthecircle(O))WeinferthequadrilateralAXHOisinscribed)AXH^+AOH^=180°(1).Ontheotherhands,thetriangleOALisisoscelesatO,soOAL^=OLA^=α.Moreover,inthetriangleOALwehaveOAL^+OLA^+AOH^=180°infer2α+AOH^=180°(2).From(1)(2)wegetAXH^=2α.FromthatsinceAXE^=12AXH^weinferOAL^=AXE^=α.ThisgivesusthatKAX^+AXE^=KAX^+OAL^=OAX^=90°Thus,KDEF.ConsidertwoteianglesXACandXBAhavethecommonangleAXC^,ACB^=BAX^.ItfollowsthatΔAXCΔBXA(a.a)AXBX=ACAB(3).Ontheotherhands,sinceADisthebisectoroftheangleA^wehaveBDDC=ABAC(4).SinceXFisthebisectoroftheAXB^,BFFA=BXAX(5).From(3)(4)(5)weobtainBDDC=BFFADF//AC.Similarly,wehaveDE//AB,soquadrilateralAEDFisaparallelogramthathastwomutualperpendiculardiagonals.ThatshowsAEDFisarhombus(q.e.d)ii)ByaboveproofwehaveO,H,Larecolinear(HmidpointofBC)andOLA^=12AXH^=KXH^=α.Therefore,quadrilateralHKXLisinscribedwhichshowsthatthecircumcircleoftriangleKXLgothroughthemidpointHofBC(q.e.d)
Commented by Aina Samuel Temidayo last updated on 07/Sep/20
Thanks.
Thanks.
Commented by Aina Samuel Temidayo last updated on 07/Sep/20
I really appreciate your efforts.
Ireallyappreciateyourefforts.
Commented by Aina Samuel Temidayo last updated on 07/Sep/20
Are you there?
Areyouthere?
Commented by Aina Samuel Temidayo last updated on 07/Sep/20
Commented by Aina Samuel Temidayo last updated on 07/Sep/20
I don′t know if you can also help with  this please?
Idontknowifyoucanalsohelpwiththisplease?
Commented by Aina Samuel Temidayo last updated on 07/Sep/20
Can you please show how DE//AB ?
CanyoupleaseshowhowDE//AB?
Commented by 1549442205PVT last updated on 07/Sep/20
because AL is bisector of BAC^(�) .
becauseALisbisectorofBAC^.
Commented by 1549442205PVT last updated on 07/Sep/20
Prove ((CE)/(EA))=((CD)/(DB)) base on the hypothesis  XE is bisector ⇒((CE)/(EA))=((XC)/(XA)),ΔXAC∽  ΔXBA⇒((XC)/(XA))=((AC)/(AB)) but ((AC)/(AB))=((CD)/(DB))(as AD is bisector)  ⇒((CE)/(EA))=((CD)/(DB))(q.e(√d))
ProveCEEA=CDDBbaseonthehypothesisXEisbisectorCEEA=XCXA,ΔXACΔXBAXCXA=ACABbutACAB=CDDB(asADisbisector)CEEA=CDDB(q.ed)
Commented by Aina Samuel Temidayo last updated on 07/Sep/20
Thanks.
Thanks.

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