2024 AIME I Problem 12

Below is the professionally curated solution for Problem 12 of the 2024 AIME I, 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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Concepts:functionabsolute valuecounting intersections

Difficulty rating: 3160

12.

Define f(x)=x12f(x) = \left||x| - \tfrac{1}{2}\right| and g(x)=x14.g(x) = \left||x| - \tfrac{1}{4}\right|. Find the number of intersections of the graphs of y=4g(f(sin(2πx)))andx=4g(f(cos(3πy))).y = 4g(f(\sin(2\pi x))) \quad \text{and} \quad x = 4g(f(\cos(3\pi y))).

Solution:

Both right-hand sides take values in [0,1],[0, 1], so every intersection lies in the unit square, and there we may write both curves using φ(u)=4u1214:\varphi(u) = 4\left||u - \tfrac{1}{2}| - \tfrac{1}{4}\right|: the first is y=φ(sin2πx)y = \varphi(|\sin 2\pi x|) and the second is x=φ(cos3πy).x = \varphi(|\cos 3\pi y|). As uu increases from 00 to 1,1, φ(u)\varphi(u) runs linearly 101011 \to 0 \to 1 \to 0 \to 1 with corners at u=14,12,34.u = \frac{1}{4}, \frac{1}{2}, \frac{3}{4}. For x[0,1],x \in [0, 1], sin2πx|\sin 2\pi x| sweeps [0,1][0, 1] monotonically 44 times, so the first graph consists of 44=164 \cdot 4 = 16 monotone arcs, each climbing or descending through the full range 0y10 \le y \le 1 within a narrow vertical strip. Likewise cos3πy|\cos 3\pi y| sweeps [0,1][0, 1] monotonically 66 times for y[0,1],y \in [0, 1], so the second graph consists of 2424 monotone arcs, each crossing the full range 0x10 \le x \le 1 within a narrow horizontal strip.

Take one arc of each graph, living in the vertical strip [a,b][a, b] and the horizontal strip [c,d].[c, d]. Inside the rectangle [a,b]×[c,d],[a, b] \times [c, d], the first arc joins the bottom edge to the top edge and the second joins the left edge to the right edge, and each is monotone, so the two arcs cross exactly once. This yields 1624=38416 \cdot 24 = 384 intersection points.

One further point hides at the corner (1,1),(1, 1), which lies on both graphs: φ(sin2π)=φ(0)=1\varphi(|\sin 2\pi|) = \varphi(0) = 1 and φ(cos3π)=φ(1)=1.\varphi(|\cos 3\pi|) = \varphi(1) = 1. Near it the first graph is y18π(1x)y \approx 1 - 8\pi(1 - x) while the second satisfies x118π2(1y)2,x \approx 1 - 18\pi^2(1 - y)^2, so the two final arcs meet at their shared endpoint (1,1)(1, 1) in addition to the transversal crossing already counted. The total is 384+1=385.384 + 1 = 385.

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