Question

 A = 4 + 2²+ 2³ + 2⁴ + ... + 2²⁰

-.- Trả lời nhanh dùm mình nha!! 

Answer 1

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

\(\Rightarrow2A=8+2^3+2^4+2^5+...+2^{21}\)

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

\(\Rightarrow A=8+2^{21}-4-2^2\)

\(\Rightarrow A=8+2^{21}-8\)

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

Vậy \(A=2^{21}\)

Answer 2

2A = 8 + 2³ + 2⁴ + 2⁵ + ... + 2²¹

2A - A = (8 + 2³ + 2⁴ + 2⁵ + ... + 2²¹) - (4 + 2² + 2³ + 2⁴ + ... + 2²⁰)

A = 8 + 2²¹ - (4 + 2²)

A = 8 + 2²¹ - (4 + 4)

A = 8 + 2²¹ - 8

A = 2²¹