Tính

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Bài 13. Tính:

a)
$$\left( \frac{-5}{7} + \frac{7}{9} + \frac{11}{13} \right) \left( \frac{3}{4} - \frac{3}{4} \right)$$
$$\frac{10}{21} + \frac{6}{27} + \frac{22}{39}$$
$$\left( \frac{2}{3} \right)$$

b)
$$3 + \frac{3}{7} + \frac{3}{11} + \frac{3}{1001}$$
$$\frac{9}{9} \frac{9}{11} + 9$$