A=7.(1/10.11+1/11.12+...+1/69.70)

A=7.(1/10-1/11+1/11-1/12+...+1/69-1/70)

A=7.(1/10-1/70)

A=7. 3/35

A= 3/5

chúc bạn học tốt nha

\(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(\Rightarrow A=7\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(\Rightarrow A=7\left(\frac{1}{10}-\frac{1}{70}\right)\)

\(\Rightarrow A=7.\frac{6}{70}=\frac{6}{10}=\frac{3}{5}\)

\(A=\frac{7}{10\cdot11}+\frac{7}{11\cdot12}+\frac{7}{12\cdot13}+...+\frac{7}{69\cdot70}\)

\(\Rightarrow7A=\frac{1}{10\cdot11}+\frac{1}{11\cdot12}+\frac{1}{12\cdot13}+...+\frac{1}{69\cdot70}\)

\(\Rightarrow7A=\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\)

\(\Rightarrow7A=\frac{1}{10}-\frac{1}{70}\)

\(\Rightarrow7A=\frac{7}{70}-\frac{1}{70}\)

\(\Rightarrow7A=\frac{3}{35}\)

\(\Rightarrow A=\frac{3}{35}:7\)

\(\Rightarrow A=\frac{3}{245}\)

\(A=\frac{1}{7}.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{68}-\frac{1}{70}\right)\)

\(A=\frac{1}{7}.\left(\frac{1}{10}-\frac{1}{70}\right)=\frac{1}{7}.\frac{3}{35}=\frac{3}{245}\)

A=\(\frac{7}{10.11}\)+\(\frac{7}{11.12}\)+\(\frac{7}{12.13}\)+...+\(\frac{7}{69.70}\)

A=\(\frac{7}{10}\)-\(\frac{7}{11}\)+\(\frac{7}{11}\)-\(\frac{7}{12}\)+\(\frac{7}{12}\)-\(\frac{7}{13}\)+...+\(\frac{7}{69}\)-\(\frac{7}{70}\)

A=\(\frac{7}{10}-\frac{7}{70}\)

A=\(\frac{7}{10}-\frac{1}{10}\)

Ạ=\(\frac{6}{10}=\frac{3}{5}\).

\(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+..+\frac{7}{69.70}\)

=\(7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+..+\frac{1}{69}-\frac{1}{70}\right)\)

=\(7.\left(\frac{1}{10}-\frac{1}{70}\right)=7.\frac{3}{35}=\frac{21}{35}\)

= 7.( \(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-....-\frac{1}{70}\))

= 7.( \(\frac{1}{10}-\frac{1}{70}\))

= 7.(\(\frac{7}{70}-\frac{1}{70}\))

= 7.\(\frac{6}{70}\)

= \(\frac{3}{5}\)

\(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(\Rightarrow A=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+....+\frac{1}{69.70}\right)\)

\(\Rightarrow A=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(\Rightarrow A=7\left(\frac{1}{10}-\frac{1}{70}\right)\)

\(\Rightarrow A=7.\frac{3}{35}\)

\(\Rightarrow A=\frac{3}{5}\)

7/10 . 11 + 7/11.12 + 7/12.13 + ... + 7/69.70

= 7 ( 1/10.11 + 1/11.12 + 1/12.13 + ... + 1/69 . 70 )

= 7 ( 1/10 - 1/11 + 1/11 - 1/12 + 1/12 - 1/13 + ... + 1/69 - 1/70 )

= 7 ( 1/10 - 1/70 )

= 7 ( 7/70 - 1/70 )

= 7 . 6/70

= 42/70 

= 3/5 

Tk nha !!

=5.167

\(\frac{7}{10.11}+\frac{7}{11.12}+...+\frac{7}{69.70}\)

\(=\frac{7}{10}-\frac{7}{11}+\frac{7}{11}-\frac{7}{12}+...+\frac{7}{69}-\frac{7}{70}\)

\(=\frac{7}{10}-\frac{7}{70}\)

\(=\frac{7}{10}-\frac{1}{10}\)\(=\frac{6}{10}=\frac{3}{5}\)

Đặt \(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(A=\frac{7}{10}-\frac{7}{11}+\frac{7}{11}-\frac{7}{12}+\frac{7}{12}-\frac{7}{13}+...+\frac{7}{69}-\frac{7}{70}\)

\(A=\frac{7}{10}-\frac{7}{70}\)

\(A=\frac{3}{5}\)

\(\frac{7}{10\times11}+\frac{7}{11\times12}+\frac{7}{12\times13}+...+\frac{7}{69\times70}\)

\(=7\times\left(\frac{1}{10\times11}+\frac{1}{11\times12}+\frac{1}{12\times13}+...+\frac{1}{69\times70}\right)\)

\(=7\times\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(=7\times\left(\frac{1}{10}-\frac{1}{70}\right)\)

\(=7\times\left(\frac{7}{70}-\frac{1}{70}\right)\)

\(=7\times\frac{6}{70}\)

\(=\frac{3}{5}\)

Chúc bạn học tốt

\(\text{Đặt }A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)

\(=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(=7.\left(\frac{1}{10}-\frac{1}{70}\right)=7.\frac{60}{700}=\frac{3}{5}\)

Vậy A = 3/5.

vãi nồi :

S=7/10.11+7/11.12+...+7/69.70

=>S= 7.(1/10.11+1/11.12+...+1/69.70)

=> S=  7. ( 1/10-1/11+...+1/69-1/70)

=> S= 7. ( 1/10 - 1/70)

=> S = 7. 3/35

=> S = 3/5

We are one_Nguyễn Ngọc Sáng

\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)

\(=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(=7.\left(\frac{1}{10}-\frac{1}{70}\right)\)

\(=7.\frac{3}{35}\)

\(=\frac{3}{5}\)

\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(=7\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)

\(=7\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(=7\left(\frac{1}{10}-\frac{1}{70}\right)=7\left(\frac{7}{70}-\frac{1}{70}\right)=7.\frac{6}{70}=\frac{6}{10}=\frac{3}{5}\)

\(A=\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)

\(A=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(A=7.\left(\frac{1}{10}-\frac{1}{70}\right)\)

\(A=7.\frac{3}{35}\)

\(A=\frac{3}{5}\)

a = 7/10 x 11 + 7/11 x 12 + 7/12 x 13 + .....+ 7/69 x 70

a = 7/10 - 7/11 + 7/11 - 7/12 + 7/12 - 7/13 + 7/13 - ......+7/69 - 7/70

a= 7/10 - 7/70

a = 3/5

\(A=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{69.70}\right)\)

suy ra \(A=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)

\(A=7.\left(\frac{1}{10}-\frac{1}{70}\right)\)

\(A=7.\left(\frac{7}{70}-\frac{1}{70}\right)\)

\(A=7.\frac{6}{70}\)

\(A=\frac{3}{5}\)