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Geometry Difficulty 5.3 AIME, harder Find the answer

Suppose ABCA B C is a triangle with incircle ω\omega, and ω\omega is tangent to BC\overline{B C} and CA\overline{C A} at DD and EE respectively. The bisectors of A\angle A and B\angle B intersect line DED E at FF and GG respectively, such that BF=1B F=1 and FG=GA=6F G=G A=6. Compute the radius of ω\omega.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let α,β,γ\alpha, \beta, \gamma denote the measures of 12A,12B,12C\frac{1}{2} \angle A, \frac{1}{2} \angle B, \frac{1}{2} \angle C, respectively. We have mCEF=90γ,mFEA=90+γ,mAFG=mAFE=180α(90+γ)=m \angle C E F=90^{\circ}-\gamma, m \angle F E A=90^{\circ}+\gamma, m \angle A F G=m \angle A F E=180^{\circ}-\alpha-\left(90^{\circ}+\gamma\right)= β=mABG\beta=m \angle A B G, so ABFGA B F G is cyclic. Now AG=GFA G=G F implies that BG\overline{B G} bisects ABF\angle A B F. Since BG\overline{B G} by definition bisects ABC\angle A B C, we see that FF must lie on BC\overline{B C}. Hence, F=DF=D. If II denotes the incenter of triangle ABCA B C, then ID\overline{I D} is perpendicular to BC\overline{B C}, but since A,I,FA, I, F are collinear, we have that ADBC\overline{A D} \perp \overline{B C}. Hence, ABCA B C is isoceles with AB=ACA B=A C. Furthermore, BC=2BF=2B C=2 B F=2. Moreover, since ABFGA B F G is cyclic, BGA\angle B G A is a right angle. Construct FF^{\prime} on minor arcGF\operatorname{arc} G F such that BF=6B F^{\prime}=6 and FG=1F^{\prime} G=1, and let AB=xA B=x. By the Pythagorean theorem, AF=BG=x236A F^{\prime}=B G=\sqrt{x^{2}-36}, so that Ptolemy applied to ABFGA B F^{\prime} G yields x236=x+36x^{2}-36=x+36. We have (x9)(x+8)=0(x-9)(x+8)=0. Since xx is a length we find x=9x=9. Now we have AB=AC=9A B=A C=9. Pythagoras applied to triangle ABDA B D now yields AD=9212=45A D=\sqrt{9^{2}-1^{2}}=4 \sqrt{5}, which enables us to compute [ABC]=12245=45[A B C]=\frac{1}{2} \cdot 2 \cdot 4 \sqrt{5}=4 \sqrt{5}. Since the area of a triangle is also equal to its semiperimeter times its inradius, we have 45=10r4 \sqrt{5}=10 r or r=255r=\frac{2 \sqrt{5}}{5}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.