Ответ:

  1. \(\frac{4x}{x-3}+\frac{2x}{x-3}=\frac{4x+2x}{x-3}=\frac{6x}{x-3}\), \(x\ne3\).
  2. \(\frac{2a+b}{a-b}+\frac{a-4b}{a-b}=\frac{2a+b+a-4b}{a-b}=\frac{3a-3b}{a-b}=3\), \(a\ne b\).
  3. \(\frac{x^2}{x+5}+\frac{25}{x+5}=\frac{x^2+25}{x+5}\), \(x\ne-5\).
  4. \(\frac{5m-1}{m^2-16}+\frac{3m+9}{m^2-16}=\frac{5m-1+3m+9}{m^2-16}=\frac{8m+8}{m^2-16}=\frac{8(m+1)}{(m-4)(m+4)}\), \(m\ne-4,4\).

Ответ: 1) \(\frac{6x}{x-3}\); 2) \(3\); 3) \(\frac{x^2+25}{x+5}\); 4) \(\frac{8(m+1)}{(m-4)(m+4)}\).