Đáp án + Giải thích các bước giải:
Ta có :
`2/2.5 + 2/5.8 + 2/8.11 + ...+2/(x.(x+3)) = 7/23`
`2/3 .( 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ...+ 1/x - 1/(x+3)) = 7/23`
`2/3 . (1/2 - 1/(x + 3)) = 7/23`
`1/2 - 1/(x+3) = 7/23 : 2/3`
`1/2 - 1/(x+3) = 7/23 . 3/2`
`1/2 - 1/(x+3) = 21/46`
`1/(x+3) = 1/2 - 21/46`
`1/(x+3) = 23/46 - 21/46`
`1/(x+3) = 1/23`
`x + 3 .1 = 1.23`
`x + 3 = 23`
`x = 23 - 3`
`x = 20`