Cách làm cho bạn:
$\frac{4\times 2\times 9}{7\times 9\times 4}= \frac{\not{4}\times 2\times \not{9}}{7\times \not{9}\times \not{4}}=\frac{2}{7}$
$\frac{5\times 8\times 17}{8\times 3\times 5}= \frac{\not{5}\times \not{8}\times 17}{\not{8}\times 3\times \not{5}}=\frac{17}{3}$
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