Maths Olympiad Prep

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Geometry Difficulty 5.4 AIME, harder Find the answer

Given one side of a triangle, as well as the radius of the circumscribed circle and the radius of the inscribed circle. What are the lengths of the other two sides? (Numerical example: a=79,R=65,r=28a=79, R=65, r=28 units.) (Try to solve without using trigonometric tables!)

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We denote the center of the circumscribed circle of the written, circumscribed circle by OO, the midpoint of BCBC by A1A_{1}, the projection of OO onto BCBC by A2A_{2}, the intersection of AOAO with BCBC and the circumscribed circle by A3A_{3} and DD, the point opposite to DD by EE. It is known that

BD=OD=CD B D=O D=C D

since, for example, the angle BOD\angle BOD and the angle OBD\angle OBD are each equal to half the sum of the angles at AA and BB of the triangle ABCABC. We can assume that ABcABc under the condition; if we omit this condition, the roles of bb and cc can naturally be swapped. The dd in (4) is the root of the equation (1). From this, we get ee based on (2), assuming that a positive number stands on the right side of (2). Therefore, the number of possible solutions can be any number between 0 and 4.

In the numerical case mentioned, (1) represents the equation

d2130d+392=0 d^{2}-130 d+39^{2}=0

from which d1=13,d2=117d_{1}=13, d_{2}=117. Accordingly, based on (2),

e12=78242854=36 e_{1}^{2}=78^{2}-4 \cdot 28 \cdot 54=36

and a negative value is obtained for e22e_{2}^{2}. Finally, from (4), we get

b1=126,c1=120 b_{1}=126, \quad c_{1}=120

Remark. In the publication, the value of aa was given as 79 due to a typographical error (the correct text appeared in the Russian and English translations). From this, we get the values b1=127.4b_{1}=127.4, and c1=117.0c_{1}=117.0 ( e22e_{2}^{2} is also negative here).

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.