1/20+1/44+1/77+1/119+1/170 Giúp mình với

Gọi `1/20 + 1/44 + 1/77 + 1/119 + 1/170` là `A`

`=> 2 × A = 2/40 + 2/88 + 2/154 + 2/238 + 2/340`

`=> 2 × A = 2/{5 × 8} + 2/{8 × 11} + 2/{11 × 14} + 2/{14 × 17} + 2/{17 × 20}`

`=> 2/3 : A = 3/{5 × 8} + 3/{8 × 11} + 3/{11 × 14} + 3/{14 × 17} + 3/{17 × 20}`

`=> 2/3 : A = 1/5 - 1/8 + 1/8 - 1/11 + 1/11 - 1/14 + 1/14 - 1/17 + 1/17 - 1/20`

`=> 2/3 : A = 1/5 - 1/20`

`=> 2/3 : A = 3/20`

`=> A = 3/20 × 2/3`

`=> A = 1/10`

Vậy `1/20 + 1/44 + 1/77 + 1/119 + 1/170 = 1/10`

`#dtkc`