Ответ:

  1. \(\frac{3x}{21y}=\frac{x}{7y}\).
  2. \(\frac{5x^2}{6x}=\frac{5x}{6}\).
  3. \(\frac{5c^4}{10c^5}=\frac{1}{2c}\).
  4. \(\frac{2m^4}{m^3}=2m\).
  5. \(\frac{16ab^4}{40ab^2}=\frac{2b^2}{5}\).
  6. \(\frac{63x^5y^4}{42x^4y^5}=\frac{3x}{2y}\).
  7. \(\frac{12a^8}{-42a^2}=-\frac{2a^6}{7}\).
  8. \(\frac{-13a^6b^5}{26a^4b^3}=-\frac{a^2b^2}{2}\).

Ответ: 1) \(\frac{x}{7y}\); 2) \(\frac{5x}{6}\); 3) \(\frac{1}{2c}\); 4) \(2m\); 5) \(\frac{2b^2}{5}\); 6) \(\frac{3x}{2y}\); 7) \(-\frac{2a^6}{7}\); 8) \(-\frac{a^2b^2}{2}\).