Đáp án + Giải thích các bước giải:
`1/3 + 1/6 + 1/10 + 1/15 + ... + 1/( x(2x + 1) ) = 11/10 (ĐK : x \ne 0 ; x \ne -1/2)`
` 2/6 + 2/12 + 2/20 + 2/30 + .... + 2/( 2x(2x + 1) ) = 11/10`
` 2 . (1/6 + 1/12 + 1/20 + 1/30 + .... + 1/( 2x(2x + 1) ) = 11/10`
` 1/(2.3) + 1/(3.4) + 1/(4.5) + ... + 1/( 2x(2x + 1) ) = 11/10 : 2`
` 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + .... + 1/(2x) - 1/(2x + 1) = 11/20`
` 1/2 - 1/(2x + 1) = 11/20`
` 1/(2x + 1) = 1/2 - 11/20`
` 1/(2x + 1) = -1/20`
`=> 2x + 1 = -20`
`=> 2x = -21`
`=> x = -21/2 (TM)`
Vậy `x = -21/2`
