2024 AIME I Problem 8

Attempt Problem 8 of the 2024 AIME I below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2024 AIME I solutions, or check the answer key.

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8.

Eight circles of radius 3434 can be placed tangent to BC‾\overline{BC} of △ABC\triangle ABC so that the circles are sequentially tangent to each other, with the first circle being tangent to AB‾\overline{AB} and the last circle being tangent to AC‾,\overline{AC}, as shown. Similarly, 20242024 circles of radius 11 can be placed tangent to BC‾\overline{BC} in the same manner. The inradius of △ABC\triangle ABC can be expressed as mn,\frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+n.m + n.

Answer: 197
Concepts:tangent circlesincircle, incenter, and inradiustrigonometry
Difficulty rating: 2560
Small Hint:

The centers sit at height ρ\rho above BC,BC, spaced 2ρ2\rho apart, and the end centers lie at distances ρcot⁡B2\rho\cot\frac{B}{2} and ρcot⁡C2\rho\cot\frac{C}{2} from BB and CC

Big Hint:

Both chains measure the same BC,BC, which determines cot⁡B2+cot⁡C2;\cot\frac{B}{2} + \cot\frac{C}{2}; the incircle satisfies BC=r(cot⁡B2+cot⁡C2)BC = r\left(\cot\frac{B}{2} + \cot\frac{C}{2}\right)

Solution:

For a chain of nn circles of radius ρ\rho tangent to BC‾,\overline{BC}, the centers lie at height ρ\rho with consecutive centers 2ρ2\rho apart. The first circle is tangent to AB‾\overline{AB} and BC‾,\overline{BC}, so its center lies on the bisector from B,B, at horizontal distance ρcot⁡B2\rho\cot\frac{B}{2} from B;B; similarly the last center is ρcot⁡C2\rho\cot\frac{C}{2} from C.C. Hence with k=cot⁡B2+cot⁡C2,k = \cot\frac{B}{2} + \cot\frac{C}{2}, BC=ρk+2ρ(n−1).BC = \rho k + 2\rho(n - 1).

The two chains give 34k+34⋅14=BC34k + 34 \cdot 14 = BC =k+2⋅2023,= k + 2 \cdot 2023, so 33k=357033k = 3570 and k=119011,k = \frac{1190}{11}, whence BC=k+4046=4569611.BC = k + 4046 = \frac{45696}{11}.

The incircle is a chain of one circle of radius r:r: BC=rk.BC = rk. Therefore r=BCk=456961190=1925,r = \frac{BC}{k} = \frac{45696}{1190} = \frac{192}{5}, and m+n=192+5=197.m + n = 192 + 5 = 197.

Problem 7#7
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