Find a mathematical formula. What is the formula for finding the number of n and the number of m that is not repeated?

Because I don't know the specific topic requirements, I tentatively asked "n independent events" and asked the probability;

Then the formula is m (m-1) ... (m-n+1)/m n = a (m, n)/m n Note: the molecule is "arrangement: the number of n arranged in the order of m".

Explanation: Molecule: the first number has m choices, that is, the second number cannot be repeated with the first number, only M- 1 choices, and so on to the nth number. Denominator: repeat n times, each time there are m choices, n times, m times.

The second question, as they say, is not repeated.

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