Đáp án:
`x=6`
Giải thích các bước giải:
`1/21+1/28+1/36+⋯+2/(x.(x+1))=2/9`
` 2(1/3.7 + 1/7.4 + 1/4.9 +...+ 1/x - 1/(x+1)) = 2/9`
` 2(1/3 - 1/7 + 1/7 - 1/4 + 1/4 - 1/9+...+ 1/x - 1/(x+1) )= 2/9`
` 2(1/3 - 1/(x+1) )= 2/9`
` 1/3 - 1/(x+1 )= 2/9 : 2 `
` 1/3 - 1/(x+1)=1/9`
`1/(x+1) = 1/3 - 1/9`
`1/(x+1)=2/9`
` x + 1.2 = 1.9 `
`x+3 = 9 `
`x=9 - 3 `
`x=6`