Đáp án:
`A = 4/(2×5) + 4/(5×8) + 4/(8×11) + ... + 4/(65×68)`
`⇔ A = 4 × [1/(2×5) + 1/(5×8) + 1/(8×11)+ ... + 1/(65×68)]`
`⇔ A = 4 × 1/3 × [1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/65 - 1/68]`
`⇔ A = 4/3 × [1/2 + ( - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/65)- 1/68]`
`⇔ A = 4/3 × [1/2 - 1/68]`
`⇔ A = 4/3 × 33/68`
`⇔ A = 11/17`
Vậy `A = 11/17`
