Question

2+2²+2³+...+2²⁰=?

Answer 1

\(A=2+2^2+...+2^{20}\\ 2A=2^2+2^3+...+2^{21}\\ A=2^{21}-2\)

Answer 2

Đặt \(A=2+2^2+2^3+...+2^{20}\)

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

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

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