Đặt \(A=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{20}}\)
\(\Rightarrow2A=2+1+\frac{1}{2}+...+\frac{1}{2^9}\)
\(\Rightarrow2A-A=\left(2+1+\frac{1}{2}+...+\frac{1}{2^9}\right)-\left(1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{10}}\right)\)
\(\Rightarrow A=2-\frac{1}{2^{10}}\)
đặt \(A=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{10}}\)
\(2A=2+1+\frac{1}{2}+...+\frac{1}{2^9}\)
\(2A-A=\left(2+1+\frac{1}{2}+...+\frac{1}{2^9}\right)-\left(1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{10}}\right)\)
\(A=2-\frac{1}{2^{10}}\)
Đặt A =1 + \(\frac{1}{2}\)+\(\frac{1}{2^2}\)...+\(\frac{1}{2^{10}}\)
Ta có : A =1 + \(\frac{1}{2}\)+\(\frac{1}{2^2}\)...+\(\frac{1}{2^{10}}\)
=> \(\frac{1}{2}\)A = \(\frac{1}{2}\)+\(\frac{1}{2^2}\)...+\(\frac{1}{2^{10}}\)+ \(\frac{1}{2^{11}}\)
=> A - \(\frac{1}{2}\)A= ( 1 + \(\frac{1}{2}\)+\(\frac{1}{2^2}\)...+\(\frac{1}{2^{10}}\) ) - ( \(\frac{1}{2}\)+\(\frac{1}{2^2}\)...+\(\frac{1}{2^{10}}\)+ \(\frac{1}{2^{11}}\))
=> \(\frac{1}{2}\)A = 1 - \(\frac{1}{2^{11}}\)
=> \(\frac{1}{2}\)A= \(\frac{2^{11}-1}{2^{11}}\)
=> A = \(\frac{2^{11}-1}{2^{10}}\)
Vậy A = \(\frac{2^{11}-1}{2^{10}}\)