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

\(=7\left(\dfrac{1}{10}-\dfrac{1}{70}\right)=\dfrac{7.60}{700}=\dfrac{420}{700}=\dfrac{3}{5}\)

\(B=\dfrac{1}{2}\left(\dfrac{1}{25}-\dfrac{1}{27}+\dfrac{1}{27}-\dfrac{1}{29}+...+\dfrac{1}{73}-\dfrac{1}{75}\right)\)

\(=\dfrac{1}{2}\left(\dfrac{1}{25}-\dfrac{1}{75}\right)=\dfrac{1}{75}\)

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

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

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

\(A=7\left(\dfrac{3}{35}\right)\)

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

Cách giải vậy đó

A=7/3(1/10-1/11+1/11-1/12+......+1/69-1/70

A=7/3(1/10-1/70)

A=7/3.3/35

A=1/5

lê mạnh tiến đạt làm sai rồi

sao lại đưa 7 ra ngoài phải đưa 7/3 ra ngoài mà

\(A=\dfrac{1}{10}+\dfrac{1}{15}+\dfrac{1}{21}+...+\dfrac{1}{120}\)

\(A=\dfrac{2}{20}+\dfrac{2}{30}+\dfrac{2}{42}+...+\dfrac{2}{240}\)

\(A=\dfrac{2}{4.5}+\dfrac{2}{5.6}+\dfrac{2}{6.7}+...+\dfrac{2}{15.16}\)

\(A=2\left(\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+...+\dfrac{1}{15.16}\right)\)

\(A=2\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+...+\dfrac{1}{15}-\dfrac{1}{16}\right)\)

\(A=2\left(\dfrac{1}{4}-\dfrac{1}{16}\right)\)

\(A=2.\dfrac{3}{16}\)

\(A=\dfrac{3}{8}\)

\(B=\dfrac{4}{3.7}+\dfrac{4}{7.11}+\dfrac{4}{11.15}+...+\dfrac{4}{107.111}\)

\(B=\dfrac{1}{3}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{15}+...+\dfrac{1}{107}-\dfrac{1}{111}\)

\(B=\dfrac{1}{3}-\dfrac{1}{111}\)

\(B=\dfrac{12}{37}\)

\(C=\dfrac{7}{10.11}+\dfrac{7}{11.12}+\dfrac{7}{12.13}+...+\dfrac{7}{69.70}\)

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

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

\(C=7\left(\dfrac{1}{10}-\dfrac{1}{70}\right)\)

\(C=7.\dfrac{3}{35}\)

\(C=\dfrac{3}{5}\)

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

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

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=\(\frac{6}{10}=\frac{3}{5}\).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

= \(\frac{3}{5}\)

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

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

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

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

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

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