Решение:
а) $\frac{1}{-2}$ = $\frac{1 * (-1)}{-2 * (-1)}$ = $\frac{-1}{2}$;
б) $\frac{1}{-3}$ = $\frac{1 * (-1)}{-3 * (-1)}$ = $\frac{-1}{3}$;
в) $\frac{-2}{-3}$ = $\frac{-2 * (-1)}{-3 * (-1)}$ = $\frac{2}{3}$;
г) $\frac{-2}{-5}$ = $\frac{-2 * (-1)}{-5 * (-1)}$ = $\frac{2}{5}$;
д) $\frac{7}{-4}$ = $\frac{7 * (-1)}{-4 * (-1)}$ = $\frac{-7}{4}$;
е) $\frac{12}{-7}$ = $\frac{12 * (-1)}{-7 * (-1)}$ = $\frac{-12}{7}$.