Ta có:
B=$\frac{36}{1.3.5}$ + $\frac{36}{3.5.7}$ + $\frac{36}{5.7.9}$ +......+ $\frac{36}{25.27.29}$
B=9.($\frac{4}{1.3.5}$ +$\frac{4}{3.5.7}$ +$\frac{4}{5.7.9}$ +.....+$\frac{4}{25.27.29}$
B=9.($\frac{1}{1.3}$ - $\frac{1}{3.5}$ +$\frac{1}{3.5}$ - $\frac{1}{5.7}$ + $\frac{1}{5.7}$ - $\frac{1}{7.9}$ +..... + $\frac{1}{25.27}$ - $\frac{1}{27.29}$ )
B=9.($\frac{1}{1.3}$ - $\frac{1}{27.29}$ )
B=9.($\frac{1}{3}$ - $\frac{1}{783}$ )
B=9.$\frac{1}{3}$ - 9.$\frac{1}{783}$
B=3-$\frac{1}{87}$