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`7/x + 4/(5.9) + 4/(9.13) + 4/(13.17) + ... + 4/(41.45) = 29/45`

`=> 7/x + (4/(5.9) + 4/(9.13) + 4/(13.17) + ... + 4/(41.45)) = 29/45`

`=> 7/x + (1/5 - 1/9 + 1/9 - 1/13 + 1/13 - 1/17 + ... + 1/41 - 1/45) = 29/45`

`=> 7/x + (1/5 - 1/45) = 29/45`

`=> 7/x + 8/45 = 29/45`

`=> 7/x = 29/45 - 8/45`

`=> 7/x = 7/15`

`=> x = 15`

Vậy `x = 16`

`----------`

`1/(3.5) + 1/(5.7) + 1/(7.9) + .... + 1/((2x+1).(2x+3)) = 15/93`

`=> 1/2 . (2/(3.5) + 2/(5.7) + 2/(7.9) + ... + 2/((2x+1).(2x+3)) = 15/93`

`=> 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/(2x+1) - 1/(2x+3) = 15/93 : 1/2`

`=> 1/3 - 1/(2x+3) = 10/31`

`=> 1/(2x + 3) = 1/3 - 10/31`

`=> 1/(2x + 3) = 1/93`

`=> 2x + 3 = 93`

`=> 2x = 93 - 3`

`=> 2x = 90`

`=> x = 90 : 2`

`=> x = 45`

Vậy `x = 45`

`#dtkc`