Question

Tính nhanh : { 2 cách } A = \(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\frac{1}{32}+\frac{1}{64}\)

 

Answer 1

Ta có: \(\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+\frac{1}{2^5}+\frac{1}{2^6}\)

=>2A=\(1+\frac{1}{2^2}+...+\frac{1}{2^4}+\frac{1}{2^5}\)

=>2A-A=(\(1+\frac{1}{2^2}+...+\frac{1}{2^4}+\frac{1}{2^5}\))--(\(\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+\frac{1}{2^4}+\frac{1}{2^5}+\frac{1}{2^6}\))

=>A=\(1-\frac{1}{2^6}\)

=>A=\(\frac{63}{64}\)

Answer 2

Nguyễn Đình Dũng chỉ được cái mồm