2026 AIME II 第 3 题

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3.

设 ABCDEABCDE 是一个非凸五边形,内角满足 ∠A=∠E=90∘\angle A = \angle E = 90^\circ 且 ∠B=∠D=45∘\angle B = \angle D = 45^\circ。已知 DE<ABDE \lt AB、AE=20AE = 20、BC=142BC = 14\sqrt{2},并且点 BB、CC、DD 都在直线 AEAE 的同一侧。还已知 ABAB 是整数,AB<2026AB \lt 2026,且五边形 ABCDEABCDE 的面积是 1616 的整数倍。求 ABAB 可能取值的个数。

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.

答案:503
知识点:坐标几何鞋带公式模运算区间内整数计数
难度评级:2510
小提示:

把 AA 放在原点,并令 E=(20,0)E = (20, 0);AA 和 EE 处的直角使 ABAB 与 EDED 竖直。

Put AA at the origin and E=(20,0);E = (20, 0); the right angles at AA and EE make ABAB and EDED vertical.

大提示:

BB 和 DD 处的 45∘45^\circ 角迫使 C=(14,AB−14)C = (14, AB - 14) 且 DE=AB−8DE = AB - 8;鞋带公式把面积条件化为关于 ABAB 的同余式。

The 45∘45^\circ angles at BB and DD force C=(14,AB−14)C = (14, AB - 14) and DE=AB−8;DE = AB - 8; the shoelace formula turns the area condition into a congruence for AB.AB.

解答:

令 A=(0,0)A = (0, 0)、E=(20,0)E = (20, 0),使五边形在直线 AEAE 上方,并设 h=ABh = AB。AA 和 EE 处的直角使 ABAB 与 EDED 竖直:B=(0,h)B = (0, h),D=(20,k)D = (20, k),其中 k=DEk = DE。在 BB 处,边 BC=142BC = 14\sqrt{2} 与向下的射线 BABA 成 45∘45^\circ 角并进入五边形,所以 C=(14,h−14)C = (14, h - 14)。类似地,在 DD 处,边 DCDC 与向下的射线 DEDE 成 45∘45^\circ 角,所以 C=(20−s,k−s)C = (20 - s, k - s),其中 s=DC2s = \frac{DC}{\sqrt{2}}。比较坐标得 s=6s = 6,k=h−8k = h - 8。此时 CC 处的内角是反角 270∘270^\circ(角和 90+45+270+45+90=54090 + 45 + 270 + 45 + 90 = 540),且 DE=h−8<ABDE = h - 8 \lt AB 自动成立。

对 A(0,0)A(0,0)、B(0,h)B(0,h)、C(14,h−14)C(14, h-14)、D(20,h−8)D(20, h-8)、E(20,0)E(20, 0) 使用鞋带公式,得到面积 [ABCDE]=12∣−14h+(−6h+168)+(−20h+160)∣=20h−164 \begin{aligned} [ABCDE] &= \frac{1}{2}\big|{-14h} + (-6h + 168) \\ &\qquad {}+ (-20h + 160)\big| \\ &= 20h - 164 \end{aligned} 20h−16420h - 164 能被 1616 整除这一条件化为 4h≡4(mod16)4h \equiv 4 \pmod{16},也就是 h≡1(mod4)h \equiv 1 \pmod 4。为了使 CC 与 BB、DD 严格在直线 AEAE 的同一侧,需要 h>14h \gt 14。

所以 hh 取 17,21,25,…,202517, 21, 25, \ldots, 2025,共有 2025−174+1=503\frac{2025 - 17}{4} + 1 = 503 个值。

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 45∘45^\circ angle with the downward ray BA,BA, heading into the pentagon, so C=(14,h−14).C = (14, h - 14). Similarly at D,D, the side DCDC makes a 45∘45^\circ angle with the downward ray DE,DE, so C=(20−s,k−s)C = (20 - s, k - s) where s=DC2.s = \frac{DC}{\sqrt{2}}. Matching coordinates gives s=6s = 6 and k=h−8.k = h - 8. The interior angle at CC is then the reflex angle 270∘270^\circ (angle sum 90+45+270+45+90=54090 + 45 + 270 + 45 + 90 = 540), and DE=h−8<ABDE = h - 8 \lt AB automatically.

The shoelace formula on A(0,0),A(0,0), B(0,h),B(0,h), C(14,h−14),C(14, h-14), D(20,h−8),D(20, h-8), E(20,0)E(20, 0) gives area [ABCDE]=12∣−14h+(−6h+168)+(−20h+160)∣=20h−164. \begin{aligned} [ABCDE] &= \frac{1}{2}\big|{-14h} + (-6h + 168) \\ &\qquad {}+ (-20h + 160)\big| \\ &= 20h - 164. \end{aligned} The condition that 20h−16420h - 164 be divisible by 1616 reduces to 4h≡4(mod16),4h \equiv 4 \pmod{16}, that is, h≡1(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 2025−174+1=503\frac{2025 - 17}{4} + 1 = 503 values.

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