Ta có 1/3x4= 1/3- 1/4
2/4x6= 1/4-1/6
..............................
5/13x18= 1/13 - 1/18
=> 1/3 -1/4 + 1/4 -1/6 +......+ 1/13 -1/18
= 1/3 - 1/18
= 5/18
=1/3-1/4+1/4-1/6+1/6-1/9+...+1/13-1/18
=1/3-1/18
=5/18
k nha
=1/3-1/4+1/4-1/6+1/6-1/9+...+1/13-1/18
=1/3-1/18
=5/18
*** tk mk tk lk***
=1/3 - 1/4 + 1/4 - 1/6 + 1/6 - 1/9 + ... + 1/13 - 1/18
=1/3 - 1/18
=5/18
*** tk mk đi
= 1/3 - 1/4 + 1/4 - 1/6 + 1/6 - 1/9 + ... + 1/13 - 1/18
= 1/3 - 1/18
= 5/1
\(\frac{1}{3.4}+\frac{1}{4.6}+\frac{3}{6.9}+\frac{4}{9.13}+\frac{5}{13.18}\)
\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{18}\)
\(=\frac{1}{3}-\frac{1}{18}\)
\(=\frac{5}{18}\)
Ai k mình mình k lại
= 1/3 - 1/4 + 1/4 - 1/6 + 1/6 - 1/9 + ... + 1/14 - 1/18
= 1/3 - 1/18
= 5/18
