2022 AMC 12A 第 25 题

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25.

一个半径为整数 rr 的圆以 (r,r)(r,r) 为圆心。对 1i141\le i\le14,有若干条互不相同、长度为 cic_i 的线段,连接点 (0,ai)(0,a_i)(bi,0)(b_i,0),并且都与该圆相切,其中 aia_ibib_icic_i 都是正整数,且 c1c2c14c_1\le c_2\le\cdots\le c_{14}。当 rr 取尽可能小的值时,c14c1\dfrac{c_{14}}{c_1} 是多少?

A circle with integer radius rr is centered at (r,r).(r,r). Distinct line segments of length cic_i connect points (0,ai)(0,a_i) to (bi,0)(b_i,0) for 1i141\le i\le14 and are tangent to the circle, where ai,a_i, bi,b_i, and cic_i are all positive integers and c1c2c14.c_1\le c_2\le\cdots\le c_{14}. What is the ratio c14c1\dfrac{c_{14}}{c_1} for the least possible value of r?r?

215\dfrac{21}{5}

8513\dfrac{85}{13}

77

395\dfrac{39}{5}

1717

答案:E
知识点:内切圆、内心与内切圆半径勾股数因数个数
难度评级:2650
解答:

圆心为 (r,r)(r,r)、半径为 rr 的圆与两条坐标轴都相切。从 (0,a)(0,a)(b,0)(b,0) 的线段满足 a2+b2=c2a^2+b^2=c^2;当 rr 等于直角边为 a,b.a,b. 的直角三角形的内切圆半径 a+bc2\tfrac{a+b-c}{2} 或半周长 a+b+c2\tfrac{a+b+c}{2} 时,该线段与圆相切。

在内切圆半径情形中,令 x=a2rx=a-2ry=b2r.y=b-2r.xy=2r2,xy=2r^2,每个正因数 xx 确定一条有方向的线段,其中 a=x+2r,a=x+2r, b=2r2/x+2r,b=2r^2/x+2r, c=x+2r2/x+2r.c=x+2r^2/x+2r. 因此恰有 d(2r2)d(2r^2) 条这样的线段。

r=1,2,3,4,5,r=1,2,3,4,5, 时,数量分别为 2,4,6,6,6,2,4,6,6,6,且半周长情形不可能,因为最小的整数直角三角形半周长为 6.6.r=6,r=6, 时,d(72)=12,d(72)=12,33-44-55 三角形再贡献两条有方向的线段。因此 66 是最小可能半径,并且恰好给出 1414 条线段。

两个半周长情形的线段满足 c=5.c=5. 在内切圆半径族中,c=x+72/x+12c=x+72/x+12x=1x=172,72, 时最大,得到 c=85.c=85. 因此 c1=5,c_1=5, c14=85,c_{14}=85,并且 c14/c1=17.c_{14}/c_1=17.

因此,正确答案是 E

The circle centered (r,r)(r,r) with radius rr is tangent to both axes. A segment from (0,a)(0,a) to (b,0)(b,0) with a2+b2=c2a^2+b^2=c^2 is tangent to it when rr equals either the inradius a+bc2\tfrac{a+b-c}{2} or the semiperimeter a+b+c2\tfrac{a+b+c}{2} of the right triangle with legs a,b.a,b.

In the inradius case, put x=a2rx=a-2r and y=b2r.y=b-2r. Then xy=2r2,xy=2r^2, and every positive divisor xx determines one oriented segment, with a=x+2r,a=x+2r, b=2r2/x+2r,b=2r^2/x+2r, and c=x+2r2/x+2r.c=x+2r^2/x+2r. Thus there are exactly d(2r2)d(2r^2) such segments.

For r=1,2,3,4,5,r=1,2,3,4,5, these counts are 2,4,6,6,6,2,4,6,6,6, and no semiperimeter case is possible because the smallest integer right triangle has semiperimeter 6.6. At r=6,r=6, d(72)=12,d(72)=12, and the 33-44-55 triangle contributes two more oriented segments. Hence 66 is the least possible radius and gives exactly 1414 segments.

The two semiperimeter segments have c=5.c=5. In the inradius family, c=x+72/x+12c=x+72/x+12 is largest at x=1x=1 or 72,72, giving c=85.c=85. Therefore c1=5,c_1=5, c14=85,c_{14}=85, and c14/c1=17.c_{14}/c_1=17.

Thus, the correct answer is E.

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