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Cho A = $\frac{1}{3}$ .$\frac{5}{7}$ .$\frac{9}{11}$ ....$\frac{4n-3}{4n-1}$ và B= $\frac{7}{5}$ .$\frac{11}{9}$ .$\frac{15}{13}$ ....$\frac{4n+3}{4n+1}$. Tính AB
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