đặt A=6/5.7+6/7.9+6/9.11+......+6/33.35

\(\Leftrightarrow A=3\left(\frac{2}{5\cdot7}+\frac{6}{7\cdot9}+...+\frac{6}{33\cdot35}\right)\)

\(\Rightarrow A=3\left(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{33}-\frac{1}{35}\right)\)

\(\Rightarrow A=3\left(\frac{1}{5}-\frac{1}{35}\right)\)

\(\Rightarrow A=3\cdot\frac{6}{35}\)

\(\Rightarrow A=\frac{18}{35}\)

\(\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{31.33}+\frac{2}{33.35}\)

\(=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+...+\frac{1}{31}-\frac{1}{33}+\frac{1}{33}-\frac{1}{35}\)

\(=\frac{1}{5}-\frac{1}{35}\)

\(=\frac{6}{35}\)

Chúc bạn học giỏi nha!!!

K cho mik vs nhé danggiahuy

đặt A=2/5.7+2/7.9+2/9.11+.....+2/31.33+2/33.35

A=1/5-1/7+1/7-1/9+1/9-1/11+.....+1/31-1/33+1/33-1/35

A=1/5-1/35

A=6/35

2/5.7+2/7.9+2/9.11+.....+2/31.33+2/33.35

\(\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}...+\frac{2}{31.33}+\frac{2}{33.35}\)

\(\Leftrightarrow\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+....\frac{1}{31}-\frac{1}{33}+\frac{1}{33}-\frac{1}{35}\)

\(\frac{1}{5}-\frac{1}{35}\)

\(=\frac{6}{35}\)

theo mình nghĩ chắc là vậy

dễ thế mà mày ko bít làm à

B=2(2/3.5 - 2/ 5.7 +....................+ 2/99.101) 

B=2(1/3.5 -2/5.7+..............+1/99.100)

B=2(1/3-1/5+1/5-.............+1/99-1/100)

B=2(1/3-1/100)

B=2.97/100

B=97/50