Đáp án:
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 . $\frac{260}{783}$
B= $\frac{160}{87}$
Vậy B=$\frac{160}{87}$
CHÚC BẠN HỌC TỐT ^^