Dưới đây là kết quả của các phép trừ phân số đã cho:

1. \( \frac{1}{6} - \frac{5}{6} = -\frac{4}{6} = -\frac{2}{3} \)
2. \( \frac{6}{13} - \frac{14}{39} = \frac{6}{13} - \frac{6}{13} = 0 \)
3. \( \frac{4}{5} - \frac{15}{18} = \frac{4}{5} - \frac{5}{6} = \frac{24 - 25}{30} = -\frac{1}{30} \)
4. \( \frac{7}{21} - \frac{9}{36} = \frac{1}{3} - \frac{1}{4} = \frac{4-3}{12} = \frac{1}{12} \)
5. \( \frac{-12}{18} - \frac{21}{18} = \frac{-33}{18} = -\frac{11}{6} \)
6. \( \frac{-3}{21} - \frac{6}{42} = -\frac{3}{21} - \frac{3}{21} = -\frac{6}{21} = -\frac{2}{7} \)
7. \( \frac{-18}{24} - \frac{15}{24} = \frac{-33}{24} = -\frac{11}{8} \)
8. \( \frac{1}{6} - \frac{2}{1} = \frac{1}{6} - 2 = \frac{1-12}{6} = -\frac{11}{6} \)
9. \( \frac{3}{5} - \frac{7}{4} = \frac{12-35}{20} = -\frac{23}{20} \)
10. \( -2 - \frac{5}{8} = -\frac{16}{8} - \frac{5}{8} = -\frac{21}{8} \)
11. \( \frac{1}{-8} - \frac{5}{9} = -\frac{1}{8} - \frac{5}{9} = -\frac{9+40}{72} = -\frac{49}{72} \)
12. \( \frac{4}{13} - \frac{-12}{39} = \frac{4}{13} + \frac{12}{39} = \frac{4}{13} + \frac{4}{13} = \frac{8}{13} \)
13. \( -\frac{1}{21} - \frac{1}{28} = -\left(\frac{4}{84} + \frac{3}{84}\right) = -\frac{7}{84} = -\frac{1}{12} \)
14. \( -\frac{3}{29} - \frac{16}{58} = -\frac{3}{29} - \frac{8}{29} = -\frac{11}{29} \)
15. \( \frac{8}{40} - \frac{-36}{45} = \frac{1}{5} + \frac{12}{15} = \frac{1}{5} + \frac{4}{5} = 1 \)
16. \( -\frac{8}{18} - \frac{15}{27} = -\frac{4}{9} - \frac{5}{9} = -\frac{9}{9} = -1 \)
17. \( \frac{13}{30} - \frac{-1}{30} = \frac{13}{30} + \frac{1}{30} = \frac{14}{30} = \frac{7}{15} \)
18. \( \frac{2}{21} - \frac{1}{28} = \frac{8 - 3}{84} = \frac{5}{84} \)
19. \( 5 - \frac{3}{4} = 5 - 0.75 = 4.25 = \frac{17}{4} \)
20. \( \frac{18}{24} - \frac{35}{-10} = \frac{18}{24} + \frac{35}{10} = \frac{3}{4} + 3.5 = 4.25 = \frac{17}{4} \)

Nếu bạn cần giải thích cho từng phép toán hay cần hỗ trợ thêm, hãy báo cho tôi biết!