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. ..