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Đáp án:+Giải thích các bước giải:

`6) 4/(2.4) + 4/(4.6) + 4/(6.8) + ... + 4/(2016.2018)`

`= (2.2)/(2.4) + (2.2)/(4.6) + (2.2)/(6.8) + ... + (2.2)/(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)`

`= 2.504/1009`

`= 1008/1009`

`7) -3/5 . 5/7 + -3/5 . 3/7 + -3/5 . 6/7`

`= -3/5 . (5/7 + 3/7 + 6/7)`

`= -3/5 . 14/7`

` = -3/5 . 2`

`= -6/5`

`8) 1/18 + 1/54 + 1/108 + ... + 1/990`

`= 1/(2.9) + 1/(6.9) + 1/(12.9) + ... + 1/(110.9)`

`= 1/9 . (1/2 + 1/6 + 1/12 + ... +1/110)`

` = 1/9 . (1/(1.2) + 1/(2.3) + 1/(3.4) + ... + 1/(10.11) )`

`= 1/9 .(1 - 1/11)`

`= 1/9 . 10/11`

`= 10/99`

`9) -7/11 . 4/9 + -7/9.7/11 + 5 7/9`

`= -7/11 . 4/9 + -7/11 . 7/9 + 52/9`

`= -7/11.(4/9 + 7/9) + 52/9`

`= -7/11.11/9 + 52/9`

`= -7/9 + 52/9`

`= 45/9 = 5`

câu cuối thì chịu ah