86.4 Chapter 4. Linear equations of the first order. Exercise 4b at page 64

86.4.1 problem 1
86.4.2 problem 2
86.4.3 problem 3
86.4.4 problem 4
86.4.5 problem 5
86.4.6 problem 6 (a)
86.4.7 problem 6 (b)