Question

Tính:

A = 4 + 2^2 + 2^3 + 2^4 + ... + 2^20

Answer 1

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

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

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

\(A=2^{21}\)

Answer 2

2A = 8+2^3+2^4+.....+2^21

2A-  A = (2^3-2^3) + ... + (2^20 - 2^20) + 2^21+ 8 - 4 - 2^2

A = 2²¹