tính giá trị biểu thức sau: A= 1/30 + 1/42 + 1/56 + 1/72 +1/90 + 1/110 + 1/132

`A=1/30+1/42+1/56+1/72+1/90+1/110+1/132`

`A=1/(5.6)+1/(6.7)+1/(7.8)+1/(8.9)+1/(9.10)+1/(10.11)+1/(11.12)`

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

`A=1/5-1/12`

`A=7/60`

Vậy `A=7/60`