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| 送交者: 车五进二 2019月09月20日11:00:01 于 [灵机一动] 发送悄悄话 |
| 回 答: 趣味的数学-87 由 gugeren 于 2019-09-19 07:53:17 |
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Let P(n) be the number of n-digit sequences such that no adjcent digits are identical, therefore P(n) = 10 * 9^(n-1). The answer to this problem is P(9) + C(8,1)*P(8) + C(7,2)*P(7) + C(6,3)*P(6) + C(5,4)*P(5) |
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