Geometry Problem. Post your solution in the comment box below.

Level: Mathematics Education, High School, Honors Geometry, College.

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## Friday, August 11, 2017

### Geometry Problem 1342: Circle, Secant, Chord, Midpoint, Concyclic Points, Cyclic Quadrilateral

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https://goo.gl/photos/GUjUkpRUbFxGVfvd8

ReplyDelete1. Consider triangles FHA and AHE

Since AC//GD so ∠ (HAF)= ∠ (AGD)= ∠ (HED)= u

Triangle FHA similar to AHE ( case AA)

So HA/HE= HF/HA = > HA ^2= HE.HF= HB.HC= HN^2

Since H is the midpoint of NA => A,N,B,C form a Harmonic conjugated points i.e (A,N,B,C)= (C,B,N,A) =- 1

2. Since M is the midpoint of BC so MB^2=MC^2= MN. MA ..( properties of Harmonic conjugated points)

Draw new circle with BC as a diameter and from A draw a tangent to this circle at T ( see sketch)

Let N’ is the projection of T over MA

Relation in right triangle MTA give MT^2= MN’.MA= MB^2= MN. MA => N’ coincide to N

Relation in right triangle MTA give AT^2= AM.AN= AB.AC = AD.AE ( power from A to circles M and O)

So D,E, M, N are cocyclic

Problem 1342

ReplyDeleteIs <HFA=<GFE=<GDE=<HAE so triangle AHF is similar with triangle EHA so AH^2=HF*HE=HB*HC=HN^2.From Newton (A,N,B,C)=-1.

So AB/BN=AC/CN or AC*BN=AB*CN or AC*(AN-AB)=AB*(AC-AN) or

AC*AN-AC*AB=AB*AC-AB*AN or 2AB*AC=AN*(AC+AB) or

2AB*AC=2AN*AM or AN*AM=AB*AC=AD*AE .

Therefore MNDE is cyclic.

APOSTOLIS MANOLOUDIS 4 HIGH SCHOOL OF KORYDALLOS PIRAEUS GREECE

Problem 1342

ReplyDeleteNeed a bit of algebraic simplification in addition to geometry.

Let MN = u, NB = v and BH = w so that

MC = u+v and AH = v+w.

Now < HAF = < FGD = < FED and so

AH is a tangent to circle AFE.

Hence (v+w)2 = HF.HE = w(2u+2v+w) which simplifies to v2 = 2uw ….(1)

Further AD.AE = AB.AC

= (v+2w)(2u+3v+2w)

= 2(v + w)(2u + 3v + 2w) – v(2u + 3v + 2w)

= 2(v + w)(u + 2v + 2w) + 2(v + w)( u + v) – v(2u + 3v + 2w)

= 2(v + w)(u + 2v + 2w) + (2uv + 2uw + 2v2 + 2vw – 2uv – 3v2 – 2vw)

= 2(v + w)(u + 2v + 2w) + (2uw – v2)

= 2(v + w)(u + 2v + 2w) from (1)

= AN.AM

Since AD.AE = AN.AM,

M,N,D,E are concyclic

Sumith Peiris

Moratuwa

Sri Lanka