nhanh nha A=9/9.19 + 9/19.29 + 9/29.39 +...+ 9/1999.2009

Đáp án:

`A = 9/(9.19) + 9/(19.29) + 9/(29.39) + ... + 9/(1999.2009)`

`A = 9.( 1/(9.19) + 1/(19.29) + 1/(29.39) + ... + 1/(1999.2009) )`

`10A= 9. ( 10/(9.19) + 10/(19.29) + 10/(29.39) + ... + 10/(1999.2009) )`

`10A= 9. ( 1/9-1/19 + 1/19 - 1/29 + 1/29 - 1/39 + ... + 1/1999 - 1/2009)`

`10A= 9. (1/9 - 1/2009)`

`10A= 9. 2000/18081`

`10A=2000/2009`

`A = 2000/2009 : 10`

`A = 2000/2009 . 1/10`

`A = 200/2009`

Vậy `A = 200/2009`