1! +2! +3! +4! = 1+2+6+24=33。

1! +2! +3! +4! +5! = 1+2+6+24+ 120= 153。

It means factorial of 5. The operation method is 5, 4, 3, 2 and 1, and the result is 120. Factorial factor is an operation symbol invented by Keyston Kaman in 1808, and it is a mathematical term.

Necessity of definition

Because the factorial of a positive integer is a continuous multiplication operation, the result of multiplying 0 by any real number is 0. Therefore, we can't generalize or deduce 0 with the definition of positive integer factorial! = 1. That is, "0! = 1"。 Give "0!" The definition is only to facilitate the expression and operation of related formulas.