8.1 problem 1837
  8.2 problem 1838
  8.3 problem 1839
  8.4 problem 1840
  8.5 problem 1841
  8.6 problem 1842
  8.7 problem 1843
  8.8 problem 1844
  8.9 problem 1845
  8.10 problem 1846
  8.11 problem 1847
  8.12 problem 1848
  8.13 problem 1849
  8.14 problem 1850
  8.15 problem 1851
  8.16 problem 1852
  8.17 problem 1853
  8.18 problem 1854
  8.19 problem 1855