Question

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

Answer 1

Lời giải lớp 11

\(S=A-1=1+2^1+2^2+...+2^{20}\)

\(\left\{{}\begin{matrix}U_1=1\\q=2\end{matrix}\right.\) \(\Rightarrow S=\dfrac{u_1\left(q^{n+1}-1\right)}{q-1}=2^{21}-1\) \(\Rightarrow A=2^{21}\)

Answer 2

A = 4+2²+2³+2⁴+....+2²⁰

Đặt B = 2²+2³+2⁴+....+2²⁰

2B = 2³+2⁴+2⁵+....+2²¹

B = 2B - B = 2²¹ - 2²

=> A = 4 + (2²¹ - 2²)

=> A = 2²+2²¹-2²

=> A = 2²¹