2011 AIME II 第 6 题

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6.

如果一个有序整数四元组 (a,b,c,d)(a, b, c, d) 满足 1a<b<c<d101 \le a \lt b \lt c \lt d \le 10a+d>b+ca + d \gt b + c,则称它为有趣的。有多少个有趣的有序四元组?

Define an ordered quadruple of integers (a,b,c,d)(a, b, c, d) to be interesting if 1a<b<c<d101 \le a \lt b \lt c \lt d \le 10 and a+d>b+c.a + d \gt b + c. How many interesting ordered quadruples are there?

答案:80
知识点:组合双射对称性
难度评级:2390
解答:

条件 a+d>b+ca + d \gt b + c 等价于 dc>bad - c \gt b - a。总共有 (104)=210\binom{10}{4} = 210 个四元组,而对合映射 (a,b,c,d)(a, b, c, d) (11d,11c,11b,11a)\small \mapsto (11 - d,\, 11 - c,\, 11 - b,\, 11 - a) 会交换外侧间隔 bab - adcd - c。因此满足 dc>bad - c \gt b - a 的四元组与满足 dc<bad - c \lt b - a 的四元组数量相等, 答案为 210T2\frac{210 - T}{2},其中 TT 是满足 dc=bad - c = b - a 的四元组数量。

ba=dc=kb - a = d - c = kcb=jc - b = j,则四元组由 (a,j,k)(a, j, k) 决定,其中 a,j,k1a, j, k \ge 1a+2k+j10a + 2k + j \le 10。当 k=1,2,3,4k = 1, 2, 3, 4 时,满足 a+j8,6,4,2a + j \le 8, 6, 4, 2 的数对 (a,j)(a, j) 分别有 282815156611 个,所以 T=50T = 50

因此有趣四元组的数量为 210502=80\frac{210 - 50}{2} = 80

The condition a+d>b+ca + d \gt b + c is equivalent to dc>ba.d - c \gt b - a. There are (104)=210\binom{10}{4} = 210 quadruples in all, and the involution (a,b,c,d)(a, b, c, d) (11d,11c,11b,11a)\small \mapsto (11 - d,\, 11 - c,\, 11 - b,\, 11 - a) exchanges the outer gaps bab - a and dc.d - c. So the quadruples with dc>bad - c \gt b - a and those with dc<bad - c \lt b - a are equinumerous, and the answer is 210T2,\frac{210 - T}{2}, where TT counts quadruples with dc=ba.d - c = b - a.

If ba=dc=kb - a = d - c = k and cb=j,c - b = j, the quadruple is determined by (a,j,k)(a, j, k) with a,j,k1a, j, k \ge 1 and a+2k+j10.a + 2k + j \le 10. For k=1,2,3,4k = 1, 2, 3, 4 the pairs (a,j)(a, j) with a+j8,6,4,2a + j \le 8, 6, 4, 2 number 28,28, 15,15, 6,6, 1,1, so T=50.T = 50.

Therefore the number of interesting quadruples is 210502=80.\frac{210 - 50}{2} = 80.

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