Desire =1* a6 =1* 25 * 24 * 23 * 22 * 21* 20 =127512000.

25

(The 6 and 25 after A are written in the upper right corner and the lower right corner of A respectively. )

In short, this is a conditional non-repetitive arrangement.

Choose 7 out of 26 numbers. And the beginning requirement is 1.

For example, there are seven boxes, the first box contains the number 1, and the next six boxes contain one of the remaining 25 numbers.

To sum up, the first box has 1 placement method, the second box has 25 kinds (because the first box has already put the number 1, only 25 numbers remain), and the third box has 24 kinds of placement methods (because the first two boxes have put two numbers, only 24). )

By analogy, it is 1 species, 25 species, 24 species, 23 species, 22 species, 2 1 species, 20 species,

Knowing the number of categories put into each box is the combination of these times, because it is not repeated and is the division of the same problem. Is the multiplication of these numbers.

There are two formulas for permutation and combination, one is permutation and the other is combination.

Detailed mathematical explanation, show you 3 websites. Better than I said.

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