84.12 Chapter 6. Exact first-order differential equations. Supplementary problems

84.12.1 problem 6.6
84.12.2 problem 6.7
84.12.3 problem 6.8
84.12.4 problem 6.9
84.12.5 problem 6.10
84.12.6 problem 6.11
84.12.7 problem 6.12
84.12.8 problem 6.13