Let ABC be a given triangle. Let D be a point on [BC] such that ∣AD∣=∣AB∣+∣AD∣∣BD∣2=∣AC∣+∣AD∣∣CD∣2 and E be a point such that D∈[AE] and ∣CD∣=∣CD∣+∣CE∣∣DE∣2. Prove that ∣AE∣=∣AB∣+∣AC∣. (Ali Doğanaksoy).
Solution
Lemma. Let ABC be a triangle with ∣AB∣2+∣AB∣⋅∣AC∣=∣BC∣2. Then m(A)=2m(C). Proof: Let D be an intersection of interior angle bisector of A with BC. Then ∣BD∣=∣AB∣⋅k, ∣CD∣=∣AC∣⋅k. ∣AB∣⋅(∣AB∣+∣BC∣)⋅k=∣BC∣2⋅k⇒∣AB∣⋅∣BC∣=∣BC∣2⋅k⇒∣AB∣=∣BC∣. Since ∣BD∣=∣AB∣⋅k, △ABD∼△CBA⇒m(BCA)=m(BAD)=21m(BAC). Done.
By the lemma, ∣AD∣2+∣AD∣⋅∣AB∣=∣BD∣2⇒m(BAD)=2m(B) and ∣AD∣2+∣AD∣⋅∣AC∣=∣CD∣2⇒m(CAD)=2m(C). Let m(B)=β and m(C)=α. Then m(A)=120∘.
∣DE∣2=∣CD∣2+∣CD∣⋅∣CE∣⇒m(DCE)=2m(DEC) and m(DCE)+m(DEC)=m(ADC)=3β⇒m(DCE)=2β and m(DEC)=β. Thus, A, B, E, C are concyclic.
Since α=60∘−β, the measure of the arc ACE is equal to 240∘−2β. Let us take a point F on the arc BEC satisfying m(BAF)=m(CAF)=60∘. The measure of the arc ABF is equal to 240∘. Therefore, ∣AF∣=∣AE∣.
m(BCF)=m(BAF)=60∘, m(CBF)=m(CAF)=60∘, m(BFC)=180∘−m(BAC)=60∘⇒△BCF is equilateral.