Đáp án:
B = $\frac{5}{11}$
Giải thích các bước giải:
B = $\frac{1}{1.3}$ + $\frac{1}{3.5}$ + $\frac{1}{5.7}$ + $\frac{1}{7.9}$ + $\frac{1}{9.11}$
2B = $\frac{2}{1.3}$+ $\frac{2}{3.5}$ + $\frac{2}{5.7}$ + $\frac{2}{7.9}$ + $\frac{2}{9.11}$
2B = 1 - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{11}$
2B = 1 - $\frac{1}{11}$
2B = $\frac{10}{11}$
B = $\frac{10}{11}$ : 2
B = $\frac{10}{11}$ . $\frac{1}{2}$
B = $\frac{5}{11}$
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