Đáp án+Giải thích các bước giải:

Bài 2:

`(1/10-1)(1/11-1)(1/12-1)...(1/2012-1)`

`=9/10xx10/11xx11/12xx...xx2010/2011xx2011/2012` 

`=(9xx10xx11xx12xx...xx2010xx2011)/(10xx11xx12xx...xx2011xx2012)`

`=9/2012`

Bài 3:

`1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110+1/132`

`=1/(3xx4)+1/(4xx5)+1/(5xx6)+1/(6xx7)+1/(7xx8)+1/(8xx9)+1/(9xx10)+1/(10xx11)+1/(11xx12)`

`=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11+1/11-1/12`

`=1/3-1/12`

`=1/4`