81.8 Chapter VII. Ordinary differential equations in two dependent variables. Exercises at page 86

81.8.1 problem 8
81.8.2 problem 9
81.8.3 problem 10
81.8.4 problem 11
81.8.5 problem 12
81.8.6 problem 13
81.8.7 problem 14
81.8.8 problem 15
81.8.9 problem 16
81.8.10 problem 17
81.8.11 problem 18
81.8.12 problem 19