Ответ:
1) \(\frac{2x}{9}\cdot\frac{3}{4y}=\frac{6x}{36y}=\frac{x}{6y}\).
2) \(\frac{7ab}{8}\cdot\frac{6c}{35a}=\frac{42abc}{280a}=\frac{3bc}{20}\).
3) \(\frac{5m}{4n}\cdot\frac{4m}{5n}=\frac{20m^2}{20n^2}=\frac{m^2}{n^2}\).
4) \(\frac{18xy}{13z}\cdot\frac{26z}{27x}=\frac{18\cdot26\cdot y}{13\cdot27}=\frac{4y}{3}\).
Ответ: 1) \(\frac{x}{6y}\); 2) \(\frac{3bc}{20}\); 3) \(\frac{m^2}{n^2}\); 4) \(\frac{4y}{3}\).
