Question

Tính:

A=\(4+2^2+2^3+2^4+...+2^{20}\)

ai trả lời nhanh nhất được tick

Answer 1

\(2A=8+2^3+2^4+...+2^{21}\)

\(\Rightarrow2A-A=2^{21}+8-\left(4+2^2\right)+\left(2^3-2^3\right)+...+\left(2^{20}-2^{20}\right)\)

\(=2^{21}\)

Answer 2

Ta đặt B=\(2^2+2^3+...+2^{20}\)(1)
=> \(2B=2^3+2^4+...+2^{21}\)(2)

Lấy (2)-(1) ta được:

B= \(2^{21}-2^2\)

Ta có: \(A=4+B\)= \(2^2+2^{21}-2^2=2^{21}\)

Vật A=\(2^{21}\)

Answer 3

\(2A=8+2^3+2^4+...+2^{21}\)

\(\Rightarrow2A-A=\left(8+2^3+2^4+.....+2^{21}\right)-\left(4+2^2+2^3+....+2^{20}\right)\)

\(\Rightarrow A=2^{21}-2^2-4=2^{21}\)