a) 4 + 2 mũ 2 + 2 miux3 + ... + 2 mũ 20 Câu dễ lấy điểm :))

Answer`:`

 `a)`

Đặt `A=4+2^2+2^3+....+2^20`

Ta có`:`

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

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

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

`A=4+2^21-2^2`

`A=4+2^21-4`

`A=(4-4)+2^21`

`A=0+2^21`

`A=2^21`

Vậy `A=2^21` hay `4+2^2+2^3+....+2^20=2^21`

`#Boy`

#andy

\[\begin{array}{l}
a = 4 + {2^2} + {2^3} + {2^4} + ... + {2^{20}}\\
 \Rightarrow 2a = 8 + {2^3} + {2^4} + {2^5} + ... + {2^{20}} + {2^{21}}\\
 \Rightarrow 2a - a = a\\
 \Rightarrow a = 4 + {2^{21}} - {2^2}\\
 \Rightarrow a = 4 + {2^{21}} - 4\\
 \Rightarrow a = {2^{21}}
\end{array}\]