Đáp án:$\frac{260}{87}$
Giải thích các bước giải:
ta có:
B= $\frac{36}{1.3.5}$ + $\frac{36}{3.5.7}$ +...+ $\frac{36}{25.27.29}$
B= 9× ($\frac{4}{1.3.5}$ + $\frac{4}{3.5.7}$ +...+ $\frac{4}{25.27.29}$ )
B=9× ($\frac{5}{1.3.5}$ - $\frac{1}{1.3.5}$ + $\frac{7}{3.5.7}$ - $\frac{3}{3.5.7}$ +...+ $\frac{29}{25.27.29}$ -$\frac{25}{25.27.29}$)
B=9× ($\frac{1}{3}$ -$\frac{1}{783}$)
B= 9× ($\frac{260}{783}$)
B= $\frac{260}{87}$