Here, this "!" It is a mathematical operation symbol, which means factorial.

Factorial factor refers to the required number obtained by multiplying 1 by 2 times 3 times 4. For example, if the required number is 4, the factorial formula is 1×2×3×4, and the product is 24, that is, the factorial of 4. ? For example, if the required number is 6, the factorial formula is 1× 2× 3×…× 6, and the product is 720, which is the factorial of 6. For example, if the required number is n, the factorial formula is 1× 2× 3× …× n, and the product obtained is x, that is, the factorial of n. The factorial of any natural number greater than 1 means:? n！ = 1×2×3×……×n？ Or? n！ =n×(n- 1)！ ? ? 5! =5*4*3*2* 1= 120。

Factorial factor is an operation symbol invented by Keyston Kramp (1760–1826) in 1808.

In addition, mathematicians define 0! = 1, so 0! = 1! . Usually, the factorial is defined in the range of natural numbers, and there is no factorial for decimals, like 0.5! ,0.65! ,0.777! It's all wrong