Maths Olympiad Prep

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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 CDCE=CDAD=BCAB=CBCA\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=1804x\angle CED = \angle BAC = 180^\circ - 4x.

Since the triangle BEDBED is isosceles, we have DEB=90x2\angle DEB = 90^\circ - \frac{x}{2}. As DEB+CED=180\angle DEB + \angle CED = 180^\circ, we get
(90x2)+(1804x)=180, (90^\circ - \frac{x}{2}) + (180^\circ - 4x) = 180^\circ,
whence follows x=20x = 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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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.