2025 AIME II Problem 12

Below is the professionally curated solution for Problem 12 of the 2025 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2025 AIME II solutions, or check the answer key.

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Concepts:law of cosinestriangle areaquadraticpattern recognition

Difficulty rating: 3160

12.

Let A1A2A11A_1A_2 \ldots A_{11} be an 1111-sided non-convex simple polygon with the following properties:

• For every integer 2i10,2 \le i \le 10, the area of AiA1Ai+1\triangle A_iA_1A_{i+1} is 1.1.

• For every integer 2i10,2 \le i \le 10, cos(AiA1Ai+1)=1213.\cos(\angle A_iA_1A_{i+1}) = \frac{12}{13}.

• The perimeter of the 1111-gon A1A2A11A_1A_2 \ldots A_{11} is equal to 20.20.

Then A1A2+A1A11A_1A_2 + A_1A_{11} can be expressed as mnpq\frac{m\sqrt{n} - p}{q} where m,m, n,n, p,p, and qq are positive integers, nn is not divisible by the square of any prime, and no prime divides all of m,m, p,p, and q.q. Find m+n+p+q.m + n + p + q.

Solution:

Let ri=A1Air_i = A_1A_i for 2i11,2 \le i \le 11, and let θ\theta be the common angle, with cosθ=1213\cos\theta = \frac{12}{13} and sinθ=513.\sin\theta = \frac{5}{13}. Each area condition says 12riri+1513=1,\frac{1}{2} r_i r_{i+1} \cdot \frac{5}{13} = 1, so riri+1=265r_i r_{i+1} = \frac{26}{5} for i=2,,10.i = 2, \ldots, 10. Consecutive products being equal forces the rir_i to alternate between two values a=r2=r4=a = r_2 = r_4 = \cdots and b=r3=r5=,b = r_3 = r_5 = \cdots, with ab=265;ab = \frac{26}{5}; in particular r11=b.r_{11} = b.

By the law of cosines, every side AiAi+1A_iA_{i+1} with 2i102 \le i \le 10 has the same length s,s, where s2=a2+b22ab1213=(a+b)22ab485=(a+b)220.s^2 = a^2 + b^2 - 2ab \cdot \tfrac{12}{13} = (a + b)^2 - 2ab - \tfrac{48}{5} = (a+b)^2 - 20. Writing u=a+b,u = a + b, the perimeter condition is 9u220+u=20.9\sqrt{u^2 - 20} + u = 20. Squaring 9u220=20u9\sqrt{u^2 - 20} = 20 - u gives 81u21620=40040u+u2,81u^2 - 1620 = 400 - 40u + u^2, which simplifies to 4u2+2u101=0,4u^2 + 2u - 101 = 0, so u=1+954u = \frac{-1 + 9\sqrt{5}}{4} (the positive root; then 20u>020 - u \gt 0 as required).

Thus A1A2+A1A11=a+b=9514,A_1A_2 + A_1A_{11} = a + b = \frac{9\sqrt{5} - 1}{4}, with 55 squarefree and no prime dividing all of 9,9, 1,1, 4.4. The answer is 9+5+1+4=19.9 + 5 + 1 + 4 = 19.

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