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Geometry Difficulty 4.5 AIME Prove it Japan

Let II and OO be the incenter and circumcenter of a triangle ABCABC, respectively. If AB=2AB = 2, AC=3AC = 3 and AIO=90\angle AIO = 90^\circ, what is the area of the triangle ABCABC? Here for a line segment XYXY its length also is denoted by XYXY.

Solution

Let MM, NN be the midpoints of the sides ABAB, ACAC, respectively. Since AMO=ANO=90=AIO\angle AMO = \angle ANO = 90^\circ = \angle AIO, we see that the points AA, MM, NN, II lie on the circle having AOAO as a diameter. In particular, the quadrilateral AMINAMIN is inscribed in this circle, and hence we have ANI=BMI\angle ANI = \angle BMI.

Since we have from the triangle inequality that BC>ABAC=1BC > |AB - AC| = 1, we can take a point DD on the side BCBC in such a way that BM=BDBM = BD holds. From BM=BDBM = BD and MBI=DBI\angle MBI = \angle DBI, we get the fact that the triangles MBIMBI and DBIDBI are congruent, and hence that BMI=BDI\angle BMI = \angle BDI.

Thus we have BDI=ANI\angle BDI = \angle ANI, from which it follows that INC=IDC\angle INC = \angle IDC. Since we also have NCI=DCI\angle NCI = \angle DCI, as II is the incenter of the triangle ABCABC, we conclude that the triangles NCINCI and DCIDCI are congruent. Hence, we have CD=CN=32CD = CN = \frac{3}{2}, and therefore, BC=BD+DC=BM+CN=1+32=52BC = BD + DC = BM + CN = 1 + \frac{3}{2} = \frac{5}{2}.

Denote by HH the foot of perpendicular line drawn from AA to the line BCBC. From 22+(52)2>322^2 + (\frac{5}{2})^2 > 3^2, we see that the point HH lies on the side BCBC. If we let BD=xBD = x, then we get from the Pythagorean theorem that 22=x2=32(52x)22^2 = x^2 = 3^2 - (\frac{5}{2} - x)^2. Solving this, we get x=14x = \frac{1}{4}. We then have AH=22116=374AH = \sqrt{2^2 - \frac{1}{16}} = \frac{3\sqrt{7}}{4} and finally, we obtain that the area of the triangle ABC=12BCAH=15716ABC = \frac{1}{2} BC \cdot AH = \frac{15\sqrt{7}}{16}.

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