1, "a": a stands for arrangement, which is the number of species arranged, and is related to the order.

2. "C": C stands for combination. Regardless of the order of the numbers, there are several ways to combine several numbers.

Second, the calculation method is different.

1, "a": the order should be considered when calculating. There are two kinds of permutation: sorted permutation and full permutation. When m different elements are taken out from n different elements, when m

2. "C": No need to consider the order when calculating. The calculation formula is

or

Third, the law is different.

1, "a": permutation with repetition is a special arrangement. You can repeatedly select m elements from n different elements. Arranging in a column in a certain order is called a repeatable arrangement of m elements starting from n elements. If and only if the selected elements are the same and the arrangement order of the elements is the same, the two arrangements are the same.

2. "C": The combination with repetiton is a special combination. You can repeatedly select m elements from n different elements. Synthesizing a group of m elements called n elements regardless of their order is a repeatable combination. Two repeated combinations are the same if and only if the elements are the same and the same elements are taken the same times.

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