Question

[ahihi]

Tính: A = 4/5.7 +  4/7.9  + 4/9.11 +  ...  + 4/21.23  + 4/23.25

Answer 1

[Dora]

`A=4/[5.7]+4/[7.9]+4/[9.11]+...+4/[21.23]+4/[23.25]`

`A=2.(2/[5.7]+2/[7.9]+....+2/[23.25])`

`A=2.(1/5-1/7+1/7-1/9+....+1/23-1/25)`

`A=2.(1/5-1/25)`

`A=2. 4/25`

`A=8/25`