Question

[phamngocson]

Tính 

a 1/1.2+1/2.3+1/3.4+.......+1/9.10

Answer 1

[Phạm Thanh Huyền]

bằng 9/10 đó bạn

* mình nha, thanks ^.^

Answer 2

[Thắng Nguyễn]

đặt A= 1/1.2+1/2.3+1/3.4+.......+1/9.10,ta có:

\(\Leftrightarrow A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)

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

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

\(\Rightarrow A=\frac{9}{10}\)