Question

Tính:

Answer 1

\(B=2(\dfrac{2}{1.3}+\dfrac{2}{3.5}+...+\dfrac{2}{9.11}) \\=2(1-\dfrac{1}{3}+...+\dfrac{1}{9}-\dfrac{1}{11}) \\=2(1-\dfrac{1}{11})=\dfrac{20}{11}\)

Answer 2

`1/2 B= 2/(1.3) + 2/(3.5) + ... + 2/(9.11)`

`1/2 B = 1/1 - 1/3 + 1/3 - 1/5 + ... + 1/9 - 1/11`

`1/2B = 10/11`

`B = 20/11`