Question

[Nguyễn Ngọc Mai]

1/2+1/4+1/8+1/16+1/32+.....+1/x=127/256

làm giúp mình với 14 phút nữa mình phải nộp rồi

ai nhanh nhất mình tick

Answer 1

[Thiên An]

\(\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}+...+\dfrac{1}{x}=\dfrac{127}{256}\)

Đặt VT là A

\(\Rightarrow2A=1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{2}{x}\)

\(2A-A=\left(1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{2}{x}\right)-\left(\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+...+\dfrac{1}{x}\right)=\dfrac{127}{256}\)

\(\Leftrightarrow A=1-\dfrac{1}{x}=\dfrac{127}{256}\)

\(\Leftrightarrow\dfrac{1}{x}=\dfrac{129}{256}\)

\(\Rightarrow x=\dfrac{256}{129}\)