3. Выполните вычитание: 1) \(\frac{x-8}{4x^2}-\frac{5-12x}{8x^3}\); 2) \(\frac{20}{a^2+4a}-\frac{5}{a}\); 3) \(\frac{m^2}{m^2-9}-\frac{m}{m+3}\); 4) \(2p-\frac{14p^2}{7p+3}\).

1. \[\frac{x-8}{4x^2}-\frac{5-12x}{8x^3}=\frac{2x(x-8)-(5-12x)}{8x^3}=\frac{2x^2-4x-5}{8x^3}.\]
2. \[\frac{20}{a^2+4a}-\frac5a=\frac{20}{a(a+4)}-\frac{5(a+4)}{a(a+4)}=\frac{-5a}{a(a+4)}=-\frac5{a+4}.\]
3. \[\frac{m^2}{m^2-9}-\frac{m}{m+3}=\frac{m^2}{(m-3)(m+3)}-\frac{m(m-3)}{(m-3)(m+3)}=\frac{3m}{m^2-9}.\]
4. \[2p-\frac{14p^2}{7p+3}=\frac{2p(7p+3)-14p^2}{7p+3}=\frac{6p}{7p+3}.\]

Ответ: 1) \(\frac{2x^2-4x-5}{8x^3}\); 2) \(-\frac5{a+4}\); 3) \(\frac{3m}{m^2-9}\); 4) \(\frac{6p}{7p+3}\).