Question

[nguyen hoai nam]

b) Cho B=4+3 mu 2+3 mu 3+3 mu 4+...+3 mu 2003+3 mu 2004 va C=3 mu 2005 so sanh B va C.

ai lam dung dau tien minh se tick cho . minh can gap lam 

Answer 1

[Trí Tiên]

Ta có : \(B=4+3^2+3^3+...+3^{2004}\)

\(=1+3+3^2+3^3+...+3^{2004}\)

\(\Rightarrow3B=3+3^2+3^3+3^4+...+3^{2005}\)

\(\Rightarrow3B-B=\left(3+3^2+3^3+...+3^{2005}\right)-\left(1+3+3^2+...+3^{2004}\right)\)

\(\Rightarrow2B=3^{2005}-1\)

\(\Rightarrow B=\frac{3^{2005}-1}{2}< 3^{2005}\)

Hay : \(B< C\)

Vậy : \(B< C\)

Answer 2

[Chu Công Đức]

Hình như sai đề hay sao đấy bạn Nam đáng lẽ 4 thành 3

Sửa lại :

\(B=3+3^2+3^3+3^4+...+3^{2003}+3^{2004}\)

\(3B=3.\left(3+3^2+3^3+3^4+...+3^{2003}+3^{2004}\right)\)

\(=3^2+3^3+3^4+3^5+...+3^{2004}+3^{2005}\)

\(3B-B=\left(3^2+3^3+3^4+3^5+...+3^{2004}+3^{2005}\right)-\left(3+3^2+3^3+3^4+...+3^{2003}+3^{2004}\right)\)

\(2B=3^{2005}-3\)

\(B=\frac{3^{2005}-3}{2}< 3^{2005}=C\)

\(\Rightarrow B< C\)