Bạn tham khảo:
- Khoảng cách là $2$ vì $3 - 1 = 2$
- Công thức: $\dfrac{4}{n(n+2)(n+4)} = \dfrac{1}{n(n+2)} - \dfrac{1}{(n+2)(n+4)}$
Bài giải:
$B = \dfrac{36}{1.3.5} + \dfrac{36}{3.5.7} + ... + \dfrac{36}{25.27.29}$
$= \dfrac{1}{9} ( \dfrac{4}{1.3.5} + \dfrac{4}{3.5.7} + ... + \dfrac{4}{25.27.29}$
$= 9 ( \dfrac{1}{1.3} - \dfrac{1}{3.5} + \dfrac{1}{3.5} + ... + \dfrac{1}{25.27} - \dfrac{1}{27.29} )$
$= 9 ( \dfrac{1}{1.3} - \dfrac{1}{27.29} )$
$= \dfrac{260}{87}$