a = 2/1.5+3/5.11+4/11.19+5/19.29+6/29.41 tính giá trị biểu thức

Đáp án:

`A = 2/(1 × 5) + 3/(5 × 11) + 4/(11 × 19) + 5/(19 × 29) + 6/(29 × 41)`

`⇔ 2 × A = 2 × (2/(1 × 5) + 3/(5 × 11) + 4/(11 × 19) + 5/(19 × 29) + 6/(29 × 41) )`

`⇔ 2 × A = 4/(1 × 5) + 6/(5 × 11) + 8/(11 × 19) + 10/(19 × 29) + 12/(29 × 41)`

`⇔ 2 × A = 1 - 1/5 + 1/5 - 1/11 + 1/11 - 1/9 + 1/19 - 1/29 + 1/29 - 1/41`

`⇔ 2 × A = 1 + (- 1/5 + 1/5 - 1/11 + 1/11 - 1/9 + 1/19 - 1/29 + 1/29) - 1/41`

`⇔ 2 × A = 1 - 1/41`

`⇔ 2 × A = 40/41`

`⇔ A = 40/41 ÷ 2`

`⇔ A=20/41`

`text{Vậy}` `A = 20/41`