\(A=\dfrac{1}{2}+\dfrac{2}{4}+\dfrac{3}{8}+...+\dfrac{10}{2^{10}}\)

\(2A=\dfrac{1}{1}+\dfrac{2}{2}+\dfrac{3}{4}+...+\dfrac{10}{2^9}\)

\(2A-A=\left(1+\dfrac{2}{2}+\dfrac{3}{4}+...+\dfrac{10}{2^9}\right)-\left(\dfrac{1}{2}+\dfrac{2}{4}+...+\dfrac{10}{2^{10}}\right)\)

\(A=1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2^9}-\dfrac{10}{2^{10}}\)

\(B=1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2^9}\)

\(2B=2+1+\dfrac{1}{2}+...+\dfrac{1}{2^8}\)

\(2B-B=\left(2+1+\dfrac{1}{2}+...+\dfrac{1}{2^8}\right)-\left(1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2^9}\right)\)

\(B=2-\dfrac{1}{2^9}\)

Suy ra \(A=B-\dfrac{10}{2^{10}}=2-\dfrac{1}{2^9}-\dfrac{10}{2^{10}}=\dfrac{509}{256}\)

a) \(\dfrac{-7}{5}\) . \(\dfrac{4}{23}\)  + \(\dfrac{4}{23}\) . \(\dfrac{2}{5}\)

= \(\dfrac{4}{23}\) . ( \(\dfrac{-7}{5}\) + \(\dfrac{2}{5}\) )

= \(\dfrac{4}{23}\) . -1

= \(\dfrac{-4}{23}\)

b) \(\dfrac{-6}{7}\) . \(\dfrac{3}{5}\) +\(\dfrac{-6}{7}\) .\(\dfrac{2}{5}\) - \(1\dfrac{1}{3}\)

= \(\dfrac{-6}{7}\) . \(\dfrac{3}{5}\) +\(\dfrac{-6}{7}\) .\(\dfrac{2}{5}\) - \(\dfrac{4}{3}\) 

= \(\dfrac{-6}{7}\) . (\(\dfrac{3}{5}\) + \(\dfrac{2}{5}\) ) - \(\dfrac{4}{3}\) 

= \(\dfrac{-6}{7}\) . 1 - \(\dfrac{4}{3}\) 

= \(\dfrac{-6}{7}\) - \(\dfrac{4}{3}\) 

= \(\dfrac{-18}{21}\) - \(\dfrac{28}{21}\)

= \(\dfrac{-18}{21}\) + \(\dfrac{-28}{21}\)

= \(\dfrac{-46}{21}\)

Đáp án là B bạn nhé 

chúc bạn học tốt

chắc là B

- Tất cả các đoạn thẳng có trong hình vẽ là : MP ; MQ ; MN ; NQ ; PQ ; NP

\(\Rightarrow\) Chọn \(B\)

_HT_

Đáp án( B ) nha

B. -120,34; 3,61;4,61.

B nhaaaa