Question

[Tô Khánh Linh]

Biết:  . n!=1.2.3...n.Tính: 1.1! + 2.2! + 3.3! + 4.4! + 5.5! = 

Answer 1

[Nguyễn Ngọc Quý]

1.1! + 2.2! + 3.3! + 4.4! + 5.5! = (2-1).1! + (3-1).2! + (4-1).3! + (5-1).4! + (6-1).5!

= 2! - 1! + 3! - 2! + 4! - 3! + 5! - 4! + 6! - 5!

= (2! - 2!) + (3! - 3!) + (4! - 4!) + (5! - 5!) + 6! - 1

= 6! - 1 = 720 - 1 = 719