Cách làm cho bạn:
a. $\frac{2}{3}\times \frac{3}{4}\times \frac{4}{5}:\frac{1}{5}=\frac{2}{3}\times \frac{3}{4}\times \frac{4}{5}\times \frac{5}{1}=\frac{2\times \not{3}\times \not{4}\times \not{5}}{\not{3}\times \not{4}\times \not{5}\times 1}=\frac{2}{1}=2$
b. $\frac{2}{5}\times \frac{3}{4}\times \frac{5}{6}:\frac{3}{4}=\frac{2}{5}\times \frac{3}{4}\times \frac{5}{6}\times \frac{4}{3}=\frac{2\times \not{3}\times \not{5}\times \not{5}}{\not{5}\times \not{4}\times 6\times \not{3}}=\frac{2}{6}= \frac{1}{3}$
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