Question

[Đỗ Tiến Dũng]

Cho biểu thức :1/2+1/2^2+1/2^3+1/2^4+...+1/2^100

Chứng tỏ A<1

Answer 1

[Trần Cao Vỹ Lượng]

\(\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...+\frac{1}{2^{100}}< \frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+...+\frac{1}{99\cdot100}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\)

\(=1-\frac{1}{100}\)

\(=\frac{99}{100}< 1\)

\(\Rightarrow\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+...+\frac{1}{2^{100}}< 1\left(đpcm\right)\)

Answer 2

[I don't need you]

ta có: \(A=\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+...+\frac{1}{2^{100}}\)

\(\Rightarrow2A=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{99}}\)

\(\Rightarrow2A-A=1-\frac{1}{2^{100}}\)

\(A=1-\frac{1}{2^{100}}< 1\)

\(\Rightarrow A< 1\left(đpcm\right)\)