Đáp án:
Giải thích các bước giải:
`1/21+ 1/28+ 1/36+...+ 2/(x. (x+ 1))= 2/9`
`2/42+ 2/56+ 2/72+...+ 2/(x. (x+ 1))= 2/9`
`2. (1/42+ 1/56+ 1/72+...+ 1/(x. (x+ 1)))= 2/9`
`(1/(6. 7)+ 1/(7. 8)+ 1/(8. 9)+...+ 1/(x. (x+ 1)))= 2/9: 2`
`(1/6- 1/7+ 1/7- 1/8+ 1/8- 1/9+...+ 1/x- 1/(x+ 1))= 1/9`
`1/6- 1/(x+ 1)= 1/9`
`1/(x+ 1)= 1/6- 1/9`
`1/(x+ 1)= 3/18- 2/18`
`1/(x+ 1)= 1/18`
`⇒ x+ 1= 18`
`x= 18- 1`
`x= 17`
Vậy `x= 17`
