*Lời giải :

Bài 1

`a) 1/(-3) + 2/5 + (-5)/2`

`= 1/15 + (-5)/2`

`= (-73)/30`

`b) 1/2 + (-3)/8 + 5/9`

`= 1/8 + 5/9`

`= 49/72`

`c) 13/(-30) + 17/45 + (-7)/18`

`= (-1)/18 + (-7)/18`

`= (-8)/18`

`d) (-5)/12 + 4/9 + (-11)/6`

`= 1/36 + (-11)/6`

`= (-65)/36`

Bài 2

`a) 2/(3 . 5) + 2/(5 . 7) + 2/(7 . 9) + 2/(9 . 11) + 2/(11 . 13) + 2/(13 . 15)`

`= 2 . (1/3 – 1/5 + 1/5 – 1/7 + 1/7 – 1/9 + 1/9 – 1/11 + 1/11 – 1/13 + 1/13 – 1/15)`

`= 2 . (1/3 – 1/15)`

`= 2 . 4/15`

`= 8/15`

`b) 2/(3 . 5) + 2/(5.  7) + 2/(7 . 9) + …. + 2/(99 . 101)`

`= 2 . (1/3 – 1/5 + 1/5 – 1/7 + 1/7 – 1/9 + … + 1/99 – 1/101)`

`= 2 . (1/3 – 1/101)`

`= 2 . 98/303`

`= 196/303`