2020 AIME II 第 13 题

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13.

凸五边形 ABCDEABCDE 的边长为 AB=5AB = 5BC=CD=DE=6BC = CD = DE = 6EA=7EA = 7。此外,该五边形有一个内切圆 (与五边形每条边都相切的圆)。求 ABCDEABCDE 的面积。

Convex pentagon ABCDEABCDE has side lengths AB=5,AB = 5, BC=CD=DE=6,BC = CD = DE = 6, and EA=7.EA = 7. Moreover, the pentagon has an inscribed circle (a circle tangent to each side of the pentagon). Find the area of ABCDE.ABCDE.

答案:60
知识点:内切圆、内心与内切圆半径切线三角恒等式
难度评级:3060
解答:

设从 A,B,C,D,EA, B, C, D, E 到内切圆的切线段长分别为 a,b,c,d,ea, b, c, d, e。则 a+b=5a + b = 5b+c=6b + c = 6c+d=6c + d = 6d+e=6d + e = 6e+a=7e + a = 7。中间的方程给出 d=bd = be=ce = c,所以 c+a=7c + a = 7;结合 a+b=5a + b = 5b+c=6b + c = 6,得到 a=3a = 3b=d=2b = d = 2c=e=4c = e = 4。若 rr 为内切圆半径,则切线段长为 tt 的顶点处内角满足 tanθ2=rt\tan\frac{\theta}{2} = \frac{r}{t},且半角之和是 540540^\circ 的一半:arctanr3+2arctanr2+2arctanr4=270. \begin{aligned} &\arctan\frac{r}{3} + 2\arctan\frac{r}{2} \\ &\quad {}+ 2\arctan\frac{r}{4} = 270^\circ. \end{aligned}

β=arctanr2\beta = \arctan\frac{r}{2}γ=arctanr4\gamma = \arctan\frac{r}{4}。则 arctanr3=2702(β+γ)\arctan\frac{r}{3} = 270^\circ - 2(\beta + \gamma),并且由于 tan(270θ)=cotθ\tan(270^\circ - \theta) = \cot\theta,得到 r3=cot2(β+γ)\frac{r}{3} = \cot 2(\beta + \gamma)。 令 T=tan(β+γ)=r/2+r/41r2/8T = \tan(\beta + \gamma) = \frac{r/2 + r/4}{1 - r^2/8} =6r8r2= \frac{6r}{8 - r^2}, 恒等式 r3=1T22T\frac{r}{3} = \frac{1 - T^2}{2T} 变为 2rT=3(1T2)2rT = 3(1 - T^2),代入 TT 并清分母得 5r484r2+64=0,5r^4 - 84r^2 + 64 = 0, 所以 r2=16r^2 = 16r2=45r^2 = \frac{4}{5}。当 r2=45r^2 = \frac{4}{5} 时,每个半角都远小于 5454^\circ,半角和远不足 270270^\circ;所以这个根是增根。因此 r=4r = 4

半周长为 s=5+6+6+6+72=15s = \frac{5 + 6 + 6 + 6 + 7}{2} = 15,所以面积为 rs=415=60rs = 4 \cdot 15 = 60

Let the tangent lengths from A,B,C,D,EA, B, C, D, E to the incircle be a,b,c,d,e.a, b, c, d, e. Then a+b=5,a + b = 5, b+c=6,b + c = 6, c+d=6,c + d = 6, d+e=6,d + e = 6, and e+a=7.e + a = 7. The middle equations give d=bd = b and e=c,e = c, so c+a=7;c + a = 7; with a+b=5a + b = 5 and b+c=6b + c = 6 this yields a=3,a = 3, b=d=2,b = d = 2, c=e=4.c = e = 4. If rr is the inradius, the interior angle at a vertex with tangent length tt satisfies tanθ2=rt,\tan\frac{\theta}{2} = \frac{r}{t}, and the half-angles sum to half of 540:540^\circ: arctanr3+2arctanr2+2arctanr4=270. \begin{aligned} &\arctan\frac{r}{3} + 2\arctan\frac{r}{2} \\ &\quad {}+ 2\arctan\frac{r}{4} = 270^\circ. \end{aligned}

Let β=arctanr2\beta = \arctan\frac{r}{2} and γ=arctanr4.\gamma = \arctan\frac{r}{4}. Then arctanr3=2702(β+γ),\arctan\frac{r}{3} = 270^\circ - 2(\beta + \gamma), and since tan(270θ)=cotθ,\tan(270^\circ - \theta) = \cot\theta, we get r3=cot2(β+γ).\frac{r}{3} = \cot 2(\beta + \gamma). With T=tan(β+γ)=r/2+r/41r2/8T = \tan(\beta + \gamma) = \frac{r/2 + r/4}{1 - r^2/8} =6r8r2,= \frac{6r}{8 - r^2}, the identity r3=1T22T\frac{r}{3} = \frac{1 - T^2}{2T} becomes 2rT=3(1T2),2rT = 3(1 - T^2), and substituting TT and clearing denominators gives 5r484r2+64=0,5r^4 - 84r^2 + 64 = 0, so r2=16r^2 = 16 or r2=45.r^2 = \frac{4}{5}. For r2=45r^2 = \frac{4}{5} every half-angle is well under 54,54^\circ, so the half-angle sum falls far short of 270;270^\circ; this root is extraneous. Hence r=4.r = 4.

The semiperimeter is s=5+6+6+6+72=15,s = \frac{5 + 6 + 6 + 6 + 7}{2} = 15, so the area is rs=415=60.rs = 4 \cdot 15 = 60.

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