How many different five-digit positive integers can be formed using the digits 1, 2, 3,4, 5,6, and 7 if none of the digits can o

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问题 How many different five-digit positive integers can be formed using the digits 1, 2, 3,4, 5,6, and 7 if none of the digits can occur more than once in the integer?

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答案2,520

解析 This example asks how many ways there are to order 5 integers chosen from a set of 7 integers. According to the counting principle above, there are(7)(6)(5)(4)(3)= 2,520 ways to do this. Note that this is equal to
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