Maths Olympiad Prep

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Geometry Difficulty 5.0 AIME, harder Find the answer Philippines

Problem:

Let II be the center of the incircle of triangle ABCABC. Suppose that this incircle has radius 33, and that AI=5AI = 5. If the area of the triangle is 20222022, what is the length of BCBC?

(a) 670
(b) 672
(c) 1340
(d) 1344

This was a multiple-choice question, but the options didn't survive into the source we have. The answer given is a, and the solution below works it through.

Solution

Solution:

Let rr be the inradius, r=3r = 3. Let SS be the area, S=2022S = 2022. Let a=BCa = BC.

Recall that S=rsS = r \cdot s, where ss is the semiperimeter. So:
s=Sr=20223=674 s = \frac{S}{r} = \frac{2022}{3} = 674

Let AIAI be the distance from AA to the incenter II. There is a formula:
AI2=bc(b+c)2[(b+c)2a2]+r2 AI^2 = \frac{bc}{(b + c)^2} \left[ (b + c)^2 - a^2 \right] + r^2
But this is complicated. Alternatively, recall that:
AI2=r2+(sa)21 AI^2 = \frac{r^2 + (s - a)^2}{1}
So:
AI2=r2+(sa)2 AI^2 = r^2 + (s - a)^2
Given AI=5AI = 5, r=3r = 3, s=674s = 674:
52=32+(674a)2 5^2 = 3^2 + (674 - a)^2
25=9+(674a)225 = 9 + (674 - a)^2
16=(674a)216 = (674 - a)^2
674a=±4674 - a = \pm 4
So a=674±4=678a = 674 \pm 4 = 678 or 670670

But aa must be less than ss (since aa is a side, ss is the semiperimeter), and aa must be positive. Both 678678 and 670670 are possible, but let's check which is correct.

If a=678a = 678, then sa=4s - a = -4, which is not possible (since sas - a is the sum of the other two sides divided by 22 and must be positive). So a=670a = 670.

Thus, the answer is 670\boxed{670}.

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