$\frac{36}{1.3.5}=$ $9.(\frac{1}{1.3}-$ $\frac{1}{3.5})$
$...$ (tách như vậy đến hết dãy)
Ta có:
$B=9.(\frac{1}{1.3}-$ $\frac{1}{3.5}+$ $\frac{1}{3.5}-$ $\frac{1}{3.7}+...+$ $\frac{1}{25.27}-$ $\frac{1}{27.29})$
⇒$B=9.(\frac{1}{3}-$ $\frac{1}{27.29})$
⇒$B=3-\frac{1}{87}$
⇒$B=\frac{260}{87}$