Ta có: A =1/2+1/4+1/8+1/16+....+1/256+1/512 

=> 2A = 1 + 1/2 + 1/4 + 1/8 + ...+ 1/128 + 1/256

=> 2A - A = (1 + 1/2 + 1/4 + 1/8 + ...+ 1/128 + 1/256 -(1/2+1/4+1/8+1/16+....+1/256+1/512 )

         A =  1 - 1/512 = 511/512

a.\(\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)...\left(1-\frac{1}{2015}\right)=\frac{1}{2}.\frac{2}{3}...\frac{2014}{2015}=\frac{1.2.3...2014}{2.3...2015}=\frac{1}{2015}\)

b.\(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{8}+...+\frac{1}{128}-\frac{1}{256}=1-\frac{1}{256}=\frac{255}{256}\)

c.\(\frac{5}{2}+\frac{5}{4}+\frac{5}{8}+...+\frac{5}{256}=5\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\right)=5.\frac{255}{256}=\frac{1275}{256}\)

d.14,35+(13,7-13,6).1=14,35+0,1.1=14,35+0,1=14,45

a.1/2015

2A= 1 + 1/2 + 1/4 + ... +1/128

2A= 1 + 1/2 + 1/4 + ... + 1/128 + 1/256 - 1/256

2A = 1 + A - 1/256

=> A = 1- 1/256 = 255/256

ban nao tra loi minh

đcm là đầu cắt moi

vì quá dễ nên mình không thể trả lời bạn được nhé!

dễ vl ko làm đc là ngu 

 A = 1/2 + 1/4 + 1/8 + ... + 1/1024

2A = 1 + 1/2 + 1/4 + ... + 1/512

2A - A = (1 + 1/2 + 1/4 + ... + 1/512) - (1/2 + 1/4 + 1/8 + ... + 1/1024)

A = 1 - 1/1024

A = 1023/1024

\(A=\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+....+\frac{1}{1024}\)

\(\Rightarrow2A=1+\frac{1}{2}+\frac{1}{4}+......+\frac{1}{512}\)

\(\Rightarrow A=2A-A=1-\frac{1}{1024}\)

\(A=\frac{1023}{1024}\)

A=\(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{512}+\frac{1}{1024}\)

2 x A=\(1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{512}\)

2 x A - A = \(\left(1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{512}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{512}+\frac{1}{1024}\right)\)

A=\(1-\frac{1}{1024}\)

A=\(\frac{1023}{1024}\)

\(A=\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\)

\(2A=1+\frac{1}{2}+\frac{1}{4}+...+\frac{1}{128}\)

\(2A-A=\left(1+\frac{1}{2}+\frac{1}{4}+...+\frac{1}{128}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\right)\)

\(A=1-\frac{1}{256}=\frac{255}{256}\)

\(B=\frac{5}{1.2}+\frac{5}{3.4}+...+\frac{5}{91.92}\)

\(B=5.\left(\frac{1}{1.2}+\frac{1}{3.4}+...+\frac{1}{91.92}\right)\)

\(B=5.\left(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{91}-\frac{1}{92}\right)\)

\(B=5.\left(\frac{1}{47}+\frac{1}{48}+...+\frac{1}{92}\right)\)

\(A=\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\)

\(A=\left(1-\frac{1}{2}\right)+\left(\frac{1}{2}-\frac{1}{4}\right)+\left(\frac{1}{4}-\frac{1}{8}\right)+...+\left(\frac{1}{128}-\frac{1}{256}\right)\)

\(A=1-\frac{1}{256}=\frac{255}{256}\)

Ta có : \(A=\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+.....+\frac{1}{256}\)

\(2A=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+.....+\frac{1}{128}\)

\(2A-A=1-\frac{1}{256}\)

\(A=\frac{255}{256}\)

a) đề sai