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Geometry Difficulty 6.2 National Olympiad Prove it Japan

A point PP is located in the interior of a triangle ABCABC. If
AP=3, BP=5, CP=2, AB:AC=2:1, BAC=60, AP = \sqrt{3},\ BP = 5,\ CP = 2,\ AB : AC = 2 : 1,\ \angle BAC = 60^\circ,
what is the value of the area of the triangle ABCABC?

Here by XYXY we represent the length of the line segment XYXY.

Solution

6+732 \frac{6+7\sqrt{3}}{2}
Take a point QQ in the opposite side from the point CC with respect to the line ABAB so as to make the triangles ABQABQ and ACPACP similar. The similarity ratio of the triangle ABQABQ to the triangle ACPACP is given by AB:AC=2:1AB : AC = 2 : 1, and so we obtain AQ=2AP=23AQ = 2AP = 2\sqrt{3}, BQ=2CP=4BQ = 2CP = 4. Also, from QAB=PAC\angle QAB = \angle PAC we get
QAP=QAB+BAP=PAC+BAP=BAC=60, \angle QAP = \angle QAB + \angle BAP = \angle PAC + \angle BAP = \angle BAC = 60^{\circ},
which, together with the fact that AQ:AP=2:1AQ : AP = 2 : 1, yield APQ=90\angle APQ = 90^{\circ} and PQ=3AP=3PQ = \sqrt{3}AP = 3.
Figure 1
From these facts, we deduce that BP2=BQ2+PQ2BP^2 = BQ^2 + PQ^2, and this in turn implies that BQP=90\angle BQP = 90^{\circ}. Consequently, we get AB2=PQ2+(AP+BQ)2=28+83AB^2 = PQ^2 + (AP + BQ)^2 = 28 + 8\sqrt{3}, and the area of the triangle ABCABC is given by
12ABACsin60=38AB2=6+732 \frac{1}{2} \cdot AB \cdot AC \cdot \sin 60^{\circ} = \frac{\sqrt{3}}{8} AB^2 = \frac{6+7\sqrt{3}}{2}

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.