13. Сократить дробь: 1) 7x/(21y); 2) 16ab/(4ac); 3) 25n³/(15n⁸).

1. \(\dfrac{7x}{21y}=\dfrac{x}{3y}\), \(y\ne0\).
2. \(\dfrac{16ab}{4ac}=\dfrac{4b}{c}\), \(a\ne0\), \(c\ne0\).
3. \(\dfrac{25n^3}{15n^8}=\dfrac{5}{3n^5}\), \(n\ne0\).

Ответ: 1) \(\dfrac{x}{3y}\); 2) \(\dfrac{4b}{c}\); 3) \(\dfrac{5}{3n^5}\).