Let M be the midpoint of side BC of an equilateral triangle ABC. The point D is on CA extended such that A is between D and C. The point E is on AB extended such that B is between A and E, and |MD|=|ME|. The point F is the intersection of MD and AB. Prove that ∠BFM=∠BME.
Problem
Source: Irmo 2018 p2 q8
Tags: equal angles, geometry, Equilateral Triangle
16.09.2018 14:48
It can be noted that ME≠MD for AE=AD. We have, AM is the external bisector of ∠EAD and ME=MD. Thus AMED is cyclic. Thus we have ∠BFM=180∘−∠FME−∠MEF=180∘−∠DAE−∠MEF=60∘−∠MEF=∠BME.
16.09.2018 19:21
Let N,P the projections of M onto CA,AB respectively; as MN=MP, with MD=ME, we get △DMN≅△EMP, which implies ∠ADM=∠AEM ( 1 ), making MADE cyclic, i.e. △ADF∼△MEF, done. Best regards, sunken rock
12.05.2021 11:11
Very easy by theorem 9 (Mr.vakili)
04.08.2023 18:25
Let angle BFM =x angle DMC=60°+x, angle ACM =60° angle MDC=60°-x let angle BME=y, angle MBE=120° angle BEM=60°-y In ∆DMC, MD/ MC=sin 60°/ sin(60°-x) In ∆BME, ME/ MB=sin 60°/ sin(60°-y) Since, MB=MC MD=ME sin 60°/ sin(60°-x)=sin 60°/ sin(60°-y) sin(60°-x) =sin(60°-y) 60°-x=60°-y x=y So, angle BFM=angle BME @KRISHIJIVI
30.08.2023 15:44
Let G be a point on line EG such that line EG is parallel to line BC and MG=ME=MD ⟹ M is the center of a circle with point E,D,G on it. extend ray EM to meet the circle at point H (Not E) We have to prove ∠BFM=∠BME but ∠BME=∠MEG=∠HEG=∠HDG also ∠MGD=∠MDG=60−∠MEG ∴ \therefore since \angle EDH =90, \angle EDM =30 \therefore \angle DME = 120 \therefore since \angle MEF = 60-\angle MEG, \angle EFM= MEG, \therefore \angle BFM =\angle BME Q.E.D I also thought this could be solved via barycentric coordinates but couldn't figure it out exactly