Question

[Duong Thi Nhuong]

Cho \(A=\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{2017^2}\)

\(B=\frac{2!}{3!}+\frac{2!}{4!}+\frac{2!}{5!}+...+\frac{2!}{2017!}\)

Chứng minh \(A+B< 2\)

Answer 1

[soyeon_Tiểubàng giải]

A=1/2^2 + 1/3^2 + 1/4^2 + ... + 1/2017^2

A < 1/1.2 + 1/2.3 + 1/3.4 + ... + 1/2016.2017

A < 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/2016 - 1/2017

A < 1 - 1/2017 < 1 (1)

B = 2!/3! + 2!/4! + 2!/5! + ... + 2!/2017!

B = 2!.(1/3! + 1/4! + 1/5! + ... + 1/2017!)

B < 2.(1/2.3 + 1/3.4 + 1/4.5 + ... + 1/2016.2017)

B < 2.(1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/2016 - 1/2017)

B < 2.(1/2 - 1/2017) < 2.1/2 = 1 (2)

Từ (1) và (2) => A + B < 2 (đpcm)