2026 AIME II Problem 3

Below is the professionally curated solution for Problem 3 of the 2026 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2026 AIME II solutions, or check the answer key.

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Concepts:coordinate geometryshoelace formulamodular arithmeticcounting integers in a range

Difficulty rating: 2510

3.

Let ABCDEABCDE be a nonconvex pentagon with internal angles A=E=90\angle A = \angle E = 90^\circ and B=D=45.\angle B = \angle D = 45^\circ. Suppose that DE<AB,DE \lt AB, AE=20,AE = 20, BC=142,BC = 14\sqrt{2}, and points B,B, C,C, and DD lie on the same side of line AE.AE. Suppose further that ABAB is an integer with AB<2026AB \lt 2026 and the area of pentagon ABCDEABCDE is an integer multiple of 16.16. Find the number of possible values of AB.AB.

Solution:

Place A=(0,0)A = (0, 0) and E=(20,0)E = (20, 0) with the pentagon above line AE,AE, and write h=AB.h = AB. The right angles at AA and EE make ABAB and EDED vertical: B=(0,h)B = (0, h) and D=(20,k)D = (20, k) with k=DE.k = DE. At BB the side BC=142BC = 14\sqrt{2} makes a 4545^\circ angle with the downward ray BA,BA, heading into the pentagon, so C=(14,h14).C = (14, h - 14). Similarly at D,D, the side DCDC makes a 4545^\circ angle with the downward ray DE,DE, so C=(20s,ks)C = (20 - s, k - s) where s=DC2.s = \frac{DC}{\sqrt{2}}. Matching coordinates gives s=6s = 6 and k=h8.k = h - 8. The interior angle at CC is then the reflex angle 270270^\circ (angle sum 90+45+270+45+90=54090 + 45 + 270 + 45 + 90 = 540), and DE=h8<ABDE = h - 8 \lt AB automatically.

The shoelace formula on A(0,0),A(0,0), B(0,h),B(0,h), C(14,h14),C(14, h-14), D(20,h8),D(20, h-8), E(20,0)E(20, 0) gives area 1214h+(6h+168)+(20h+160)=20h164.\frac{1}{2}\left|{-14h} + (-6h + 168) + (-20h + 160)\right| = 20h - 164. The condition 1620h16416 \mid 20h - 164 reduces to 4h4(mod16),4h \equiv 4 \pmod{16}, that is, h1(mod4).h \equiv 1 \pmod 4. For CC to lie strictly on the same side of line AEAE as BB and D,D, we need h>14.h \gt 14.

So hh runs over 17,21,25,,2025,17, 21, 25, \ldots, 2025, which is 2025174+1=503\frac{2025 - 17}{4} + 1 = 503 values.

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