`1/3.4 + 1/4.5 + 1/5.6 +...+ 1/(x(x+1)) = 3/10`
`=> 1/3 -1/4 + 1/4 - 1/5 +....+ 1/x - 1/(x+1) =3/10`
`=> 1/3 - 1/(x+1) = 3/10`
`=> 1/(x+1) = 1/3 - 3/10`
`=> 1/(x+1) =1/30`
`=> x + 1=30`
`=> x= 30-1`
`=> x= 29`
Vậy `x= 29`
Bài 2:
a) `(x+ 1,5) : 3 1/5 =-5/8`
`=> (x+1,5) : 16/5 = -5/8`
`=> x+1,5 = -5/8 . 16/5`
`=> x + 1,5= -2`
`=> x= -2 -1,5`
`=> x=-3,5`
Vậy `x= -3,5`
$\\$
b)` (8 4/5 x - 50) : 0,4 = 51`
`=> (44/5 x - 50) : 0,4 = 51`
`=> 44/5 x- 50 = 51 . 0,4`
`=> 44/5 x - 50 =102/5`
`=> 44/5 x =102/5 + 50`
`=> 44/5 x =352/5`
`=> x= 352/5 : 44/5`
`=> x= 352/5 . 5/44`
`=> x=8`
Vậy `x=8`
