gaasspppppppppppppppppppppppppppppp:((((

Đáp án+Giải thích các bước giải:

`a)` `A = 1/10 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110`

`= 1/10 + 1/(5.6) + 1/(6.7) + 1/(7.8) + 1/(8.9) + 1/(9.10) + 1/(10.11)`

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

`=1/10+1/5-1/11`

`=1/10+6/55`

`= 23/110`

`b) B = 1/(2.5) + 1/(5.8) + 1/(8.11) + ... + 1/(32.35)`

`= 1/3(3/(2.5) +3/(5.8) + 3/(8.11) + ... + 3/(32.35))`

`= 1/3(1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/32 - 1/35)`

`= 1/3(1/2 - 1/35)`

`= 1/3 . 33/70`

`= 11/70`