Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Find the answer

An isosceles trapezoid ABCDA B C D with bases ABA B and CDC D has AB=13,CD=17A B=13, C D=17, and height 3. Let EE be the intersection of ACA C and BDB D. Circles Ω\Omega and ω\omega are circumscribed about triangles ABEA B E and CDEC D E. Compute the sum of the radii of Ω\Omega and ω\omega.

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

Solution

Let Ω\Omega have center OO and radius RR and let ω\omega have center PP and radius MM. Let QQ be the intersection of ABA B and OEO E. Note that OEO E is the perpendicular bisector of ABA B because the trapezoid is isosceles. Also, we see OEO E is the circumradius of Ω\Omega. On the other hand, we know by similarity of AEB\triangle A E B and CED\triangle C E D that QE=1313+173=13303Q E=\frac{13}{13+17} \cdot 3=\frac{13}{30} \cdot 3. And, because BQ=13/2B Q=13 / 2 and is perpendicular to OQO Q, we can apply the Pythagorean theorem to OQB\triangle O Q B to see OQ=R2(132)2O Q=\sqrt{R^{2}-\left(\frac{13}{2}\right)^{2}}. Since OE=OQ+QE,R=13303+R2(132)2O E=O Q+Q E, R=\frac{13}{30} \cdot 3+\sqrt{R^{2}-\left(\frac{13}{2}\right)^{2}}. Solving this equation for RR yields R=133039R=\frac{13}{30} \cdot 39. Since by similarity M=1713RM=\frac{17}{13} R, we know R+M=3013RR+M=\frac{30}{13} R, so R+M=39R+M=39.

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