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CroatianCompetitions

Geometry Difficulty 5.2 AIME, harder Prove it Croatia

In the triangle ABCABC with ∣AB∣=∣AC∣|AB| = |AC| angle bisector of ∠ABC\angle ABC meets AC‾\overline{AC} in DD. If ∣BC∣=∣BD∣+∣AD∣|BC| = |BD| + |AD|, find all the angles. (Canada 1996)

Solution

Figure 1

Since BDBD is the bisector of the angle ∠CBA\angle CBA, it follows that ∣CD∣:∣AD∣=∣BC∣:∣AB∣|CD| : |AD| = |BC| : |AB|, and because of that ∣CD∣∣CE∣=∣CD∣∣AD∣=∣BC∣∣AB∣=∣CB∣∣CA∣\frac{|CD|}{|CE|} = \frac{|CD|}{|AD|} = \frac{|BC|}{|AB|} = \frac{|CB|}{|CA|}.

Triangles ABCABC and EDCEDC have a common angle in vertex CC and equal ratios of corresponding sides so they are similar. Hence ∠CED=∠CAB\angle CED = \angle CAB.

Let ∠ABC=2x\angle ABC = 2x. Then ∠CBD=∠DBA=x\angle CBD = \angle DBA = x and ∠ACB=2x\angle ACB = 2x. Furthermore, ∠CED=∠BAC=180∘−4x\angle CED = \angle BAC = 180^\circ - 4x.

Since the triangle BEDBED is isosceles, we have ∠DEB=90∘−x2\angle DEB = 90^\circ - \frac{x}{2}. As ∠DEB+∠CED=180∘\angle DEB + \angle CED = 180^\circ, we get
(90∘−x2)+(180∘−4x)=180∘, (90^\circ - \frac{x}{2}) + (180^\circ - 4x) = 180^\circ,
whence follows x=20∘x = 20^\circ. The angles of the observed triangle are ∠ABC=∠ACB=40∘\angle ABC = \angle ACB = 40^\circ and ∠BAC=100∘\angle BAC = 100^\circ.

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