2022 AIME I Problem 15

Attempt Problem 15 of the 2022 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 2022 AIME I solutions, or check the answer key.

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15.

Let x,x, y,y, and zz be positive real numbers satisfying the system of equations 2x−xy+2y−xy=1\sqrt{2x - xy} + \sqrt{2y - xy} = 1 2y−yz+2z−yz=2\sqrt{2y - yz} + \sqrt{2z - yz} = \sqrt{2} 2z−zx+2x−zx=3.\sqrt{2z - zx} + \sqrt{2x - zx} = \sqrt{3}. Then [(1−x)(1−y)(1−z)]2\left[(1 - x)(1 - y)(1 - z)\right]^2 can be written as mn,\frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+n.m + n.
Answer: 33
Concepts:system of equationstrigonometric identitysubstitution
Small hint:
Factor each radical as x(2−y)\sqrt{x(2 - y)} and substitute x=2sin⁡2α,x = 2\sin^2\alpha, y=2sin⁡2β,y = 2\sin^2\beta, z=2sin⁡2γz = 2\sin^2\gamma
Big hint:
Each equation collapses by the addition formula to 2sin⁡(α+β)=1,2\sin(\alpha + \beta) = 1, etc.; solve the little linear system for the angles and use 1−2sin⁡2θ=cos⁡2θ1 - 2\sin^2\theta = \cos 2\theta
Solution:
Each radicand factors: 2x−xy=x(2−y),2x - xy = x(2 - y), and so on, so 0<x,y,z≤2.0 \lt x, y, z \le 2. Substitute x=2sin⁡2α,x = 2\sin^2\alpha, y=2sin⁡2β,y = 2\sin^2\beta, z=2sin⁡2γz = 2\sin^2\gamma with α,β,γ∈(0∘,90∘].\alpha, \beta, \gamma \in \left(0^\circ, 90^\circ\right]. Then x(2−y)=4sin⁡2αcos⁡2β\sqrt{x(2 - y)} = \sqrt{4\sin^2\alpha\cos^2\beta} =2sin⁡αcos⁡β,= 2\sin\alpha\cos\beta, and each equation collapses by the sine addition formula: 2sin⁡(α+β)=1,2\sin(\alpha + \beta) = 1, 2sin⁡(β+γ)=2,2\sin(\beta + \gamma) = \sqrt{2}, 2sin⁡(γ+α)=3.2\sin(\gamma + \alpha) = \sqrt{3}.
One admissible choice is α+β=30∘,\alpha + \beta = 30^\circ, β+γ=45∘,\beta + \gamma = 45^\circ, γ+α=60∘,\gamma + \alpha = 60^\circ, which gives α=22.5∘,\alpha = 22.5^\circ, β=7.5∘,\beta = 7.5^\circ, γ=37.5∘.\gamma = 37.5^\circ. The only other branch consistent with 0∘<α,β,γ≤90∘0^\circ \lt \alpha,\beta,\gamma \le 90^\circ uses pairwise sums 150∘,135∘,120∘,150^\circ,135^\circ,120^\circ, giving α=67.5∘,\alpha = 67.5^\circ, β=82.5∘,\beta = 82.5^\circ, γ=52.5∘;\gamma = 52.5^\circ; its product below is the negative of the first branch’s product, so the requested square is identical. For the first branch, the double-angle identity gives 1−x=cos⁡2α=cos⁡45∘,1 - x = \cos 2\alpha = \cos 45^\circ, 1−y=cos⁡15∘,1 - y = \cos 15^\circ, and 1−z=cos⁡75∘.1 - z = \cos 75^\circ.
Therefore (1−x)(1−y)(1−z)=22cos⁡15∘⋅sin⁡15∘=22⋅sin⁡30∘2=28, \begin{aligned} &(1 - x)(1 - y)(1 - z) \\ &= \frac{\sqrt{2}}{2}\cos 15^\circ \\ &\quad {}\cdot \sin 15^\circ \\ &= \frac{\sqrt{2}}{2} \cdot \frac{\sin 30^\circ}{2} \\ &= \frac{\sqrt{2}}{8}, \end{aligned} whose square is 264=132.\frac{2}{64} = \frac{1}{32}. Thus m+n=1+32=33.m + n = 1 + 32 = 33.
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