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Geometry Difficulty 8.2 Shortlist Prove it Turkey

Let ABCABC be a given triangle. Let DD be a point on [BC][BC] such that AD=BD2AB+AD=CD2AC+AD|AD| = \frac{|BD|^2}{|AB| + |AD|} = \frac{|CD|^2}{|AC| + |AD|} and EE be a point such that D[AE]D \in [AE] and CD=DE2CD+CE|CD| = \frac{|DE|^2}{|CD| + |CE|}. Prove that AE=AB+AC|AE| = |AB| + |AC|. (Ali Doğanaksoy).

Solution

Lemma. Let ABCABC be a triangle with AB2+ABAC=BC2|AB|^2 + |AB| \cdot |AC| = |BC|^2. Then m(A^)=2m(C^)m(\widehat{A}) = 2m(\widehat{C}).
Proof: Let DD be an intersection of interior angle bisector of A^\widehat{A} with BCBC. Then BD=ABk|BD| = |AB| \cdot k, CD=ACk|CD| = |AC| \cdot k. AB(AB+BC)k=BC2kABBC=BC2kAB=BC|AB| \cdot (|AB| + |BC|) \cdot k = |BC|^2 \cdot k \Rightarrow |AB| \cdot |BC| = |BC|^2 \cdot k \Rightarrow |AB| = |BC|. Since BD=ABk|BD| = |AB| \cdot k, ABDCBAm(BCA^)=m(BAD^)=12m(BAC^)\triangle ABD \sim \triangle CBA \Rightarrow m(\widehat{BCA}) = m(\widehat{BAD}) = \frac{1}{2}m(\widehat{BAC}). Done.

By the lemma, AD2+ADAB=BD2m(BAD^)=2m(B^)|AD|^2 + |AD| \cdot |AB| = |BD|^2 \Rightarrow m(\widehat{BAD}) = 2m(\widehat{B}) and AD2+ADAC=CD2m(CAD^)=2m(C^)|AD|^2 + |AD| \cdot |AC| = |CD|^2 \Rightarrow m(\widehat{CAD}) = 2m(\widehat{C}). Let m(B^)=βm(\widehat{B}) = \beta and m(C^)=αm(\widehat{C}) = \alpha. Then m(A^)=120m(\widehat{A}) = 120^\circ.

DE2=CD2+CDCEm(DCE^)=2m(DEC^)|DE|^2 = |CD|^2 + |CD| \cdot |CE| \Rightarrow m(\widehat{DCE}) = 2m(\widehat{DEC}) and m(DCE^)+m(DEC^)=m(ADC^)=3βm(DCE^)=2βm(\widehat{DCE}) + m(\widehat{DEC}) = m(\widehat{ADC}) = 3\beta \Rightarrow m(\widehat{DCE}) = 2\beta and m(DEC^)=βm(\widehat{DEC}) = \beta. Thus, AA, BB, EE, CC are concyclic.

Since α=60β\alpha = 60^\circ - \beta, the measure of the arc ACE^\widehat{ACE} is equal to 2402β240^\circ - 2\beta. Let us take a point FF on the arc BEC^\widehat{BEC} satisfying m(BAF^)=m(CAF^)=60m(\widehat{BAF}) = m(\widehat{CAF}) = 60^\circ. The measure of the arc ABF^\widehat{ABF} is equal to 240240^\circ. Therefore, AF=AE|AF| = |AE|.

m(BCF^)=m(BAF^)=60m(\widehat{BCF}) = m(\widehat{BAF}) = 60^\circ, m(CBF^)=m(CAF^)=60m(\widehat{CBF}) = m(\widehat{CAF}) = 60^\circ, m(BFC^)=180m(BAC^)=60BCFm(\widehat{BFC}) = 180^\circ - m(\widehat{BAC}) = 60^\circ \Rightarrow \triangle BCF is equilateral.

Ptolemy's cyclic quadrilateral theorem applied to ABFCABFC yields: ABCF+ACBF=AFBC|AB| \cdot |CF| + |AC| \cdot |BF| = |AF| \cdot |BC|. Therefore, AB+AC=AF|AB| + |AC| = |AF| (since BF=CF=BC|BF| = |CF| = |BC|). Thus, AB+AC=AE|AB| + |AC| = |AE|. Done.

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