bài 1 tính: a) 3/1.4+3/4.7+3/7.10+…..+3/40.43 b) 4/2.4+4/4.6+4/6.8+….+4/2016.2018 c)1/18+1/54+1/108+….+1/990

`a)`

    `3/1.4 + 3/4.7 + 3/7.10 + … + 3/40.43`

`=  1- 1/4 + 1/4 – 1/7 + 1/7 – 1/10 + … + 1/40 – 1/43`

`= 1 – 1/43`

`= 43/43- 1/43`

`= 42/43`

`b)`

`4/2.4 + 4/4.6 + 4/6.8 +… + 2/2016.2018`

`= 2 . ( 2/2.4 + 2/4.6 + 2/6.8 + … + 2/2016.2018)`

`= 2 . ( 1/2 – 1/4 + 1/4 – 1/6 + 1/6 – 1/8 + … + 1/2016 – 1/2018)`

`= 2 . (1/2 – 1/2018)`

`= 2 . (1009/2018 – 1/2018)`

`= 2 . 1004/1009`

`=2008/1009`

`c)`

`1/18 + 1/54 + 1/108 + … + 1/990`

`=1/3.6 + 1/6.9 + 1/9.12 + …  +1/30.33`

`=1/3 . (1/3 – 1/6 +1/6  -1/9 +1/9 – 1/12 + … + 1/30  -1/33)`

`=1/3 .(11/33 – 1/33)`

`=1/3 . 10/33`

`= 10/99`