Đặt \(A=\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^n}\)
\(2A=2\left(\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^n}\right)\)
\(2A=1+\frac{1}{2}+...+\frac{1}{2^{n-1}}\)
\(2A-A=\left(1+\frac{1}{2}+...+\frac{1}{2^{n-1}}\right)-\left(\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^n}\right)\)
\(A=1-\frac{1}{2^n}< 1\)với mọi n -->Đpcm