Gọi `2/{9 × 12} + 2/{12 × 15} + ... + 2/{258 × 261}` là `A`
`A = 2/3 × ( 1/{9 × 12} + 1/{12 ×15} + ... + 1/{258 × 261} )`
`A = 2/3 × ( 1/9 - 1/12 + 1/12 - 1/15 + ... + 1/258 - 1/261 )`
`A = 2/3 × ( 1/9 - 1/261 )`
`A = 2/3 × 28/261`
`A = 56/783`
Vậy `A = 56/783`
`#dtkc`