12. \(\frac{x-5}{x^2-49}+\frac{12}{x^2-49}\)

\[\frac{x-5}{x^2-49}+\frac{12}{x^2-49}=\frac{x+7}{(x-7)(x+7)}=\frac1{x-7}.\]

Ответ: \(\frac1{x-7}\).