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Geometry Difficulty 4.3 AIME Prove it Ireland

The lengths of the sides of a triangle are consecutive integers and its inradius is 44. Find the lengths of the sides and the circumradius.

Solution

Let aa, bb, cc be the sides such that a=b1a = b - 1 and c=b+1c = b + 1. Recall Heron's Formula and two other well known formulae for the area of a triangle:
ABC=s(sa)(sb)(sc)=abc4R=rs, |ABC| = \sqrt{s(s-a)(s-b)(s-c)} = \frac{abc}{4R} = rs,
where r=4r = 4 is the inradius and RR the circumradius.
We obtain s=((b1)+b+(b+1))/2=3b/2s = ((b - 1) + b + (b + 1))/2 = 3b/2 and so ABC=rs=6b|ABC| = rs = 6b. Therefore, 36b2=ABC2=s(sa)(sb)(sc)=3b2(b24)/1636b^2 = |ABC|^2 = s(s - a)(s - b)(s - c) = 3b^2(b^2 - 4)/16, i.e. 192=b24192 = b^2 - 4 and so b21=195b^2 - 1 = 195. This gives b2=196=142b^2 = 196 = 14^2 and so b=14b = 14, a=13a = 13 and c=15c = 15. Finally, 6b=ABC=abc4R6b = |ABC| = \frac{abc}{4R} and so
R=abc24b=b2124=19524=658. R = \frac{abc}{24b} = \frac{b^2 - 1}{24} = \frac{195}{24} = \frac{65}{8}.

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