( `3/5` + `4/10`) : ( `x` + 1) = `1/9` x ( `\frac{1}{1.2}` + `\frac{1}{2.3}` +`\frac{1}{4.3}` + .... + `\frac{1}{9.10}`
( `3/5` + `4/10`) : ( `x` + 1) = `1/9` x ( `1/1` - `1/2` + `1/2` - `1/3` + `1/3` - `1/4` + .... + `1/9` - `1/10`)
( `3/5` + `4/10`) : ( `x` + 1) = `1/9` x ( `1/1` - `1/10`)
`1` : ( `x` + 1) = `1/9` x `9/10`
`1` : ( `x` + `1`) = `1/10`
( `x` + 1) = `1` : `1/10`
`x` + 1 = `10`
`x` = `10` - `1`
`x` = `9`
⇔ Chọn đáp án D : 9
